Active Devices for Chapter Amplifiers and Oscillators · – Basic common emitter amplifier ... Chapter 7 • Active Devices for Amplifiers and Oscillators 7–6 ECE 3001 ... •

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Active Devices for Amplifiers and OscillatorsIntroduction
• Active electronic circuits is the focus of this chapter

• The outcomes from this chapter will be:

– The use of a diode for envelope detection in AM receivers

– Basic common emitter amplifier design for RF/IF/Audioapplications

– The use of LC feedback networks around an amplifier tocreate an RF oscillator to serve a

– Creating a mixer in combination with an amplifier andoscillator for use in a superheterodyne receiver

– An integrated circuit audio amplifier capable of driving aspeaker or ear buds

Chapt

7

ECE 3001 Electronic Projects 3–1

Chapter 7 • Active Devices for Amplifiers and Oscillators

Diodes

• A diode is a two terminal device that allows current to flowin just one direction

• A silicon diode is a sandwich of n-type and p-type silicon

anode

cathode

Small signal silicon Small signal germanium Small signal Schottky

Light emitting diodewith +5v bias and a110 ohm current limiting resistor

Flat edge on case isthe cathode

Band is thecathode onthe first threediodes

+5v

Gnd

P

N

• Silicon doped to make a semiconductor

• Doping with phosphorous creates n-type or negative-charge carrier (elect.)

• Doping with boron creates p-type orpositive-charge carrier (hole)

C

A

7–2 ECE 3001 Electronic Projects

Diodes

• Diodes take various forms depending upon the application

• The current versus voltage relationship for an ideal diodeallows current flow in only one direction depending upon thepolarity of the applied voltage

• In LTspice you can work with a diode model .dc and .tran

• Consider a DC sweep to obtain the I-V characteristic

V

I

00 V0

0 vtPIV

Diode breaks downat peak inverse voltage

reverse biasregion

forward biasregion

junction thresholdvoltage

Ideal Diode Piecewise Model of Actual Diode

I

0.7v

Note: The IN4148 israted at 10mA; thissweep exceeds themaximum ratings!

The activeanalysis isDC sweep



Piecewise Diode Model Circuit Analysis

• In diode circuit analysis it is convenient to think of the diodeas a switch that follows the piecewise model, only simplifiedeven further

– When the applied voltage (anode to cathode) is above the diode acts as a perfect conductor

– When the applied voltage is below the diode acts as anopen circuit

•

Example 3.1: Simple Clipping Circuit

• Consider the following:

• For silicon diodes, the threshold voltage where the diodebegins to conduct is approximately 0.7 V (0.3 V for germa-nium and 0.2 V for Schottky)

• Diode On: Using the piecewise modeling approach you firstconsider , which means the diode is on and conductingwith current

vt

vt

Silicon smallsignal diode

id vd t +

–

vi t

vt

vi vt


Diodes

(7.1)

and

(7.2)

– The voltage across the diode is held at

• Diode Off: For the diode acts like an open circuit, sono current flows, meaning there is no voltage drop across ,thus

(7.3)

• In summary,

(7.4)

• Running an LTspice simulation produces

– The actual diode model makes the clipping rounded

id t vi t vt–

R---------------------=

vR t vi t vt–=

vtvi vt

R

vd t vi t =

vo t vd t vt, vi t vt

vi t , otherwise

= =

In this clipping application, it appears like the threshold is more like 0.6v



LTspice and the Parts Library: When working with activedevices you may not find a model in the LTspice library that youneed

• For the radio circuits the 1N34a small signal germaniumdiode is a favorite because it has a low 0.3 threshold voltage

• This part is not in the LTspice library

• What do?

– Look for device models on the Internet

– Learn how to work with external model files

• LTspice prefers that you place new models in the installfolder of LTspice

– On a shared machine with security this is a problem

– We will keep models in the same folder as the schematic

• 1N34a model placed in same folder as the schematic thatuses it


Diodes

Example 3.2: Using an External Model Library

• The diode clipper revisited to compare the 1N4148 with the1N34a

• In the above see that the clipping level drops from 0.6 to 0.3vwith the 1N34a

– This is good for use in an envelope detector (up next)

• There are many other configurations of diodes found in prac-tical electronic circuit design [1]:

Device name needs tomatch the name in the model file exactly!

1N34a



– Voltage dropper

– Voltage regulator

– Envelope detector (diode & resistor only)

0.6v 1N4148

1N4148

1N4148

+

–

0.6v+

–

0.6v+

–

+5v

5 1.8v–3.2v=

• Step voltage in diode drops of

• Adjust current with

vtRL

IL5 1.8–RL

----------------=IL

• Similar to dropper, but now youestablish voltage from ground up

• sets the current flow to besplit between the diodes and theload, e.g. a transistor base-emitterjunction in a radio circuit

Rs

1.8v

1N4148

1N4148

1N4148

Is Id IL+=

ILId

+5v

• For the diode is on and theoutput is

• For the diode is off and

• Summary

vi vtvo vi vt–=

vi vtvo 0=

vo t vi t vt,– vi t vt

0, vi t vt

=

vi vodiode dropwhen on


Diodes

– Transient Protection

– Diode clamp (DC restorer)

– Diode clipper, asymmetrical and symmetrical

– Diode on/off switch

– Half-wave and full-wave rectifier

– Simple logic gates

vi vo

• Also known as a DC restorer since theseries capacitor allows the signal to floatand bias voltage on the diode is used toset the clamp point

• Here the diode connected to ground,clamps the bottom of the waveform tozero v

• Reverse the diode to clamp the top

vt–

Make this point afixed bias voltageto change the clampvalue, positive ornegative

• Similar to the single diode clippershown earlier

• With two diodes clipping at occurs on the positive and negativesides of a waveform

vt

vi vo



Example 3.3: Diode clamping in LTspice

• In this example you will see a diode clamp in action

• To actually clamp the bottom of the waveform to zero youneed to tie the diode to v

– Note here the input signal was arbitrarily biased to -2v

DC bias

vt–Transient ascap charges

Clamping to ~0v


Diodes

Envelope Detector

• To demodulate and amplitude modulated (AM) carrier signal(look back in Chapter 5 for a review) the envelope detector,including both R and C in parallel, is used

• In simple math terms the ideal envelope detector strips off thetop or the bottom of the AM waveform and then lowpass fil-ters the signal

• For an ideal diode all is well, but with present issues ariseas shown (and demoed) in an Excel spreadsheet file

• In the above I assume a 1N34a diode (0.3v threshold) and a65% modulation index ( )

• Increasing the carrier amplitude would help, but this morechallenging under weak signal conditions

• Note: Broadcast AM can have modulation index up to 100%

vt

Modulation closeto being clipped

a 0.65=



Example 3.4: Envelope Detection with the 1N34a Diode

• Consider the following LTspice schematic:

• The AM signal is fed directly into the envelope detector:

– The above results look nice, but the modulation index(again recall from Chapter 5) is set to 50% ( )

– AM broadcasting pushes this up to 100% ( )

• Two key design considerations are:

– Is the AM signal large enough to turn on the diode withonly minimal distortion/clipping of the message signal

?

Envelope Detector

a 0.5=

a 1=

m t


Diodes

(7.5)

– Is the time constant of the RC lowpass filter circuitformed by placing R and C in parallel following the diodeable to pass the message signal and filter out the carriersignal components?

(7.6)

• Testing on the Analog Discovery

vAM valley peak vt or higher

W1

2R4C1-------------------- fc« «

To study changes interactivelymove the 10k POT into thecircuit and remove the 3.3kfixed resistor

10nf

3.3k

1N34a

Set up AWG 1



• Using the fixed resistor the waveforms are:

• Notice from the orange trace of the AM signal the height ofthe valley at 80% modulation depth falls short of reaching0.3v to fully turn on the 1N34a (this is a challenge)

– The larger input signal would help

Transistors

• A transistor is a three terminal device that can be used to con-trol or amplify a signal

• A voltage or current applied to the control terminal of thedevice allows you to control the flow of current between theremaining to terminals

• The two major families of transistors are: bipolar and field-effect (FET)


Transistors

• In integrated circuit (IC) design FETs are the dominant tran-sistor in use

• For single (to a few) device amplifier circuits, of primaryinterest in this course, bipolar devices are however preferredas they offer greater voltage gain and greater current to theload

• Six types and their symbols:

• With the NPN device a sandwich is formed with a thin layerof p-type material between two n-type layers

Metal oxide semiconductorBipolar Junction Transistors (BJTs)

Junction FET

Main focus

FET (MOSFET)

emitter

collector

base

source

gate

drainsource

gate

gategate

source source

drain

draindrain

emitter

collector

base

collector(c)

emitter(e)

base(b)

N NPV+Gnd

Bias

ic

ib

ie

• With a positive bias of about0.6v on the base-emitter junc-tion current can flow from thecollector to the emitter, suchthat

(7.7)

where is the current gain

ic ib=

Bipolar NPN TransistorSandwich

A little like two diodes



• Note: For the above equation to hold the voltage from collec-tor to emitter, must be at least 0.2V and the diode formedbetween base and emitter must be turned on, meaning thebase to emitter voltage is –0.7V

• Also note that although the collector-emitter junction whenviewed as a diode is back-biased, current however flows oncethe base-emitter junction is turned on

• Finally note: Kirchoff’s current law (KCL) also tells you that

(7.8)

• To put the BJT to work you need DC biasing circuitry plus away to superimpose an input signal and later extract it

A BJT Voltage Amplifier [2]

• At the systems level a voltage amplifier takes input andreturns output , such that

(7.9)

where is the voltage gain

– Note: is typically also a function of the operating fre-quency, but here for simplicity I assume it is a constant

– In the above is part of the source signal generator and is part of the load where is received

vce

vbe 0.6

ie ic ib+ 1 + ib= =

vs t vo t

vo t Avvs t =

AvAv

BJT

RL+–vs t

Rs

vo t vi t +

_

+

_

io t io t

RsRL vo t


Transistors

• The three basic amplifier topologies of the BJT (simplifiedform, no biasing shown) are shown below

• The common emitter (CE) is the most popular and the oneexplored in Lab 4

• The common base (CB) is shows up in radio circuits, and infact is found in the oscillator/mixer problem of Lab 4

Biasing and Coupling [2, 1]

• To make a bipolar device into an amplifier you need to biasthe device into the active region, then connect input and out-put terminals

• The significance of the active region and how to make a CEvoltage amplifier is explored next

• More detailed BJT terminal equations are

(7.10)

Common Emitter (CE) Common Collector (CC) Common Base (CB)

Vcc Vcc Vcc

vi

vi

vovi

vo

vo

VCE Vcc RcIseVBE VT

–= IC IseVBE VT

=



where is the thermal voltage (~25mv at room tempera-ture), is the saturation current (device specific), and isthe collector supply voltage

• Using a DC sweep LTspice produces the following:

• The blue curve is versus , which shows the BJT hasthree distinct operating regions:

– Cutoff (no current flow)

– Active (controlled current flow via or )

– Saturation (maximum current flow)

VTIs Vcc

Small deviationabout Q at base

Large deviationabout Q at collector

2N3904

At quiescentpoint Q,

In

Outslope ~-92 V/V

Bias device toquiescent/operatingpoint Q

Amplify smallsignal input

Separate amplifiedsignal at collectorfrom DC bias usingcoupling capacitor

Cutoff Active Saturation

Simplified inputbias network

VCE VBE

VBE IB


Transistors

• For amplifiers of interest in this course, you want to stay inthe active region at all times (known as class A)

• With DC bias only the device sits at the quiescent point Q

• When the input signal, , is also present, i.e., superimposed along with bias, both and deviate about Qand an output/input voltage gain is exists

• The slope of the versus curve at Q is the small sig-nal voltage gain (here ~-92) as alluded to in (7.9)

– Note: You can show [2] that

• Biasing is generally provided using a resistor network sur-rounding the device, and the use of coupling (series) capaci-tors to superimpose the input and extract the output

• Consider a CE voltage amplifier:

• Here the four resistors of the bias network are chosen to setthe quiescent point Q

– Note: The emitter resistor is needed to make biasing stable

vbevBE vCE

vCE vBE

Av IC VT Rc–=

vi t

vo t

VBEVE

VC

VCE

Coupling cap Coupling cap

4 - biasing resistors

IR1

IR2

IB

IC

IE Voltages and CurrentsUsed for DC Bias Analysis

AC signalinput

AC signaloutput

+

––



over temperature

• DC circuit analysis, equations (7.7) and (7.8), the threshold, e.g., 0.6 or 0.7 V, and enable the design

Example 3.5: A small signal biasing analysis example

• Let F and k, k,, and k

• Assume that

• Find the voltages and currents around the NPN device

• From ohms law

(7.11)

• Assume the current through the bias network – is largecompared with the base current, thus

• With this assumption you can easily write using ohm’s lawthat

(7.12)

• The base voltage, from the voltage divider relationship is just

v (7.13)

VBE

C1 C2 1= = R1 30= R2 10=Re 470= Rc 2.4=

100=

IR1IR2

IB+=

R1 R2IR1

IR2

IR1

VccR1 R2+------------------

510 30+ k

-------------------------- 0.125= = =

VB 510

10 30+------------------ 1.25= =


Transistors

• Assuming the base-emitter junction is biased on, with a 0.6V drop, V and

mA (7.14)

• Since and a little algebra yields

(7.15)

so

mA (7.16)

and

V (7.17)

• Note: V

• An exact analysis of the bias circuit is possible using aThevenin equivalent (notes Chapter 4 p. 4–10) of the –base biasing network

VBE VE 1.25 0.6– 0.65= =

IEVERe------

0.65470---------- 1.38= = =

IE IB IC+= IC IB=

IE1--- 1+ IC= IC

IE1 1+-------------------

1 +------------IE= =

IC100101--------- 1.38 1.37= =

VC Vcc RcIC– 5 1.1k 1.37mA – 3.49= = =

VCE 3.49 0.65– 2.84 0.2= =

R1 R2

RBB R1||R2=

VBB VccR2

R1 R2+------------------= VBE

IE

IC

Vcc

IB



• With some algebra

(7.18)

• A practical design consideration is to choose to be about

Example 3.6: Interactive CE Biasing Using Excel

• In this example I use the Excel spreadsheet CommonEmit-ter.xlsx to interactively explore biasing options

• Start with the resistor values used in Example 3.5

IEVBB VBE–

Re RBB 1 + +------------------------------------------------=

VB VBE IERe+=

IC IE

1 +------------=

VC Vcc ICRc–=

VCVcc 2


Transistors

• Suppose you want to raise the up to V

• Interactively increase in small steps corresponding to thestandard 5% resistors values (see Appendix A)

• Setting k makes V

Small Signal Characteristics

• With biasing set the small signal characteristics of the ampli-fier are now of interest

• You know the voltage gain is of interest, but there areother amplifier attributes of interest:

– = the input resistance

– = the output resistance

– = the open circuit voltage gain

VC Vcc 2 2.5=

R2

R2 9.1= VC 2.422=

Av

Rin

RoAvo



– = the voltage gain from source to load

• To focus on just the small signal model of the CE amplifierconsider the following [2]:

– Note: The above circuit contains a current controlled cur-rent source (CCCS) to represent the flow of collector cur-rent in terms of emitter current using gain

• The internal emitter resistor is a function of the quiescentemitter current and the thermal voltage

(7.19)

• Summarizing the results (see [2] for more details)

(7.20)

and

Gv

ie

1 +------------=

Ro

Rin

Rs

vsre

ReReb

Ce

BJT

B

E

C

Rc RL

+

–

vo

Optional emitterbypass capacitorto increase gain.Set Reb = 0 for max gain

ie

ib

CCCS

CCCS = current controlledcurrent source

1 + =

reIE VT

reVTIE------=

Rin 1 + re Re+ =

Ro Rc=


Transistors

(7.21)

where , the BJT transconductance, is the slopeof the versus curve at the operating point Q

• Observe that the gain contains is thedenominator

– The internal emitter resistance is typically small, 20

– For bias stability reasons the external emitter resistor canrange from say 470 up to 4700 , thus serious gainreduction can occur

– To combat gain degeneration it is common to employ abypass capacitor or a resistor in series with a capacitor

– Bias stability is maintained yet the superimposed smallsignal is then shunted to ground allowing to bereplaced by or even 0

• All of the math modeling details above are brought into anExcel worksheet to allow easy analysis of the CE amplifier,both with and without the emitter bypass capacitor

AvogmRc–

1 Re re +-----------------------------

gmRc–

1 gmRe+----------------------=

Av–

1 +------------

Rc||RLre Re+-----------------=

Gv –Rc ||RL

Rs 1 + re Re+ +---------------------------------------------------=

gm IC VT=IC vBE

Gv 1 + re Re+

ReReb



Example 3.7: Interactive CE Bias and Gain Using Excel

• Beyond the equations, the a small signal input sinusoid canbe applied and saturation and device saturation and cutoffeffects represented in the waveform as clipping

• Use the same starting point resistor values as in Example 3.7apply a 10mV sinusoid at 2kHz

• The gain is only -4.61 or in dB

• To invoke a bypass capacitor you reduce to say 0

• The gain increases to 41dB (very good), however the input

Gain is rather lowfor a radio receiver

No bypass

20log10 4.61 13.28=

Reb


Transistors

resistance, drops from 49.5k to 2k (here so OK)

• If the input signal level goes up clipping on the waveformbottom will become a problem, yet the top has plenty ofheadroom

• Adjusting the bias resistor from 10k to 8.2k (a 5% stan-dard value) works well to center the waveform

• The input level is increased to 25mV and the output is fillingthe min to max voltage range nicely

• A third tab of the Excel workbook (not shown here) showsthe output after the coupling capacitor, hence the output iscentered about 0V

Rin

Clipping due tosaturation juststarting

Very high gain now!

Rs 0=

R2



Oscillators

• The subject of oscillators means circuit design for waveformgeneration

• In radio receiver circuits the need for an oscillator most oftenarises in a superheterodyne receiver where a local oscillatoris needed to convert to via a mixing operation (seenotes Chapter 5 for more details)

• For a fixed frequency oscillator using a quartz crystal as aresonator is a popular option1

With R2 = 8.2kan input of 25mvis OK

fRF fIF

A 16 MHzcrystal


Transistors

– A crystal oscillator in combination with a phase-lockedloop (PLL) frequency synthesizer chip is used to create adigitally programmable local oscillator

• In this course a variable frequency oscillator design with ananalog interface is needed, hence an LC resonator, with Cbeing the primary adjustable element, is the objective here

• An oscillator begins as an amplifier with feedback

• Using block diagram algebra in the s-domain you can writethat [2]

(7.22)

• The condition for oscillation to occur is making the denomi-nator zero, that is

(7.23)

• If the oscillator circuit satisfies (7.23) then without anyapplied input oscillations can begin (a little bit of noise willhelp) and be sustained

1.https://en.wikipedia.org/wiki/Crystal_oscillator

AmplifierGain A(s)

FeedbackNetwork Bf(s)

xs t xo t

xf t

xi t

Xo s Xs s A s 1 A s Bf s –---------------------------------=

A s Bf s 1=

or A j2f0 Bf j2f0 1 0=



• A single BJT Colpitts (1918 patent) oscillator

– For tuning using a single capacitor, the Colpitts oscillatoris not appropriate; using a single inductor yes but that willnot work with the available components

– In the Colpitts circuit above, resonance corresponds to theLC tank formed by in parallel with the series capacitorcombination of and , thus you expect the oscillationfrequency to be about:

Long startup transient

Needed for oscillator modeling

zoom

Note here the output is notvery sinusoidal, hence filteringwill be needed.

Common collector amp configuration

InBias

L1C1 C5


Transistors

(7.24)

• A tapped inductor form of LC oscillator, invented by Hartleyin 1915, is an option that uses a single tuning capacitor

– The tapped inductor is evident and in this case the coils arecoupled so mutual inductance comes into play, that is

f01

2 L1

C1C5

C1 C5+-------------------

-----------------------------------------=

1

2 1505–10 100

12–10----------------------------------------------------------------- 1.838 MHz==

Common emitter amp configurationCoupled coils

Fast start

zoom



(7.25)

where is the coupling coefficient

– Here the total inductance is

(7.26)

– Excluding other loading capacitance in the circuit, youexpect

(7.27)

– The simulated oscillator has an of only 956 kHz

• For Lab 4 and the AM superheterodyne receiver, a commonbase oscillator using transformer feedback is employed:

Ltotal L1 L2 k L1L2+ +=

k

L 50 50 1 50 50+ + 3 50 150H= = =

f01

2 LC------------------=

1

2 10012–10 150

6–10----------------------------------------------------------------- 1.84 MHz==

f0

42IF100Osc.Trans.

1

2

3

4

TuningCap

6Common base amp configuration


Transistors

– Unfortunately this circuit does not (yet) work in LTspicesimulation, but it works very well on the breadboard!

– Note the 60pF fixed capacitor next to the tapped inductoris replaced with a 0–360 variable cap

Mixer and 1st IF Amplifier

• In a superheterodyne receiver the local oscillator and mixerare connected together, with the mixer output driving a band-pass filter (IF filter)

• The intent of the mixer is to take as inputs

(7.28)

and produce the frequency difference signal

(7.29)

• Here you choose so

• A practical mixer produces many more terms than just thesum and difference, but the IF bandpass filter can fortunatelyextract the term of interest

IF BPF

Oscillator

Antennawith Tuning

Cap

MixerReceivedSignal(s) fRF

fLO

fIF

Synchronous tuning

xRF t A 2fRFt cos=

xLO t B 2fLOt cos=

xIF t AB2

------- 2 fLO fRF t cosAB2

------- 2fIFt cos= =

fLO fRF– fIF= fLO fRF fIF+=



• A simple yet practical mixer can be an amplifier that com-bines (sums) two inputs and then via non-ideal characteris-tics, such as clipping, is able to form output terms that are thesum of the input raised to an integer power, i.e.,

, (7.30)

where is a gain scale factor that applies to each of theterms in the summation

• As long as proper mixing action occurs, that is

(7.31)

• The last line above contains the term of interest, namely

(7.32)

• The real proof of this will be in the lab when you see the mix-ing action in real-time using the Analog Discovery scope-based spectrum analyzer

xmixer t gn A 2fRFt cos B 2fLOt cos+ n

n 1=

=

gn

g2 0

cos cos+ 2 cos 2 cos 2+=

+ 2 coscos

2 coscos + cos – cos+=


Transistors

• Putting it all together, oscillator + mixer + IF BPF yields:

• The breadboard circuit:

1

2

3

4

6

42IF100Osc.Trans.

42IF1011st IFTrans.

Local Oscillator Mixer & 1st IF Amp

LO to Mixer From AD AWGAM signal

42IF10042IF101

Notice: Red and yellow tuningslug tops for ‘100’and ‘101’ respectively

Ground

+5V

Oscillatortest point oncollector

AM signalinput fromAD AWG

IF amp/mixeroutput at transformersecondary

Local Oscillator Mixer & IF Amp

Q1 Q2

C2

C5

C9

C10

R2

R1 C8

C1D1

D2

R3

C6

C4

4 & 6

4 & 6

1, 2 & 3

1, 2 & 3c c

bbe e

Wires to tuning cap



• Oscillator Test:

– The test point signal amplitude does vary over frequency,with it becoming smaller at the low end, just before theoscillator stops working

• To test the mixer and oscillator together you input an AMmodulated carrier at 1 MHz

The oscillator has a largetuning. In the currentbreadboard configurationit stops oscillating at the lowend before reaching the maximum capacitance

Tuned around1.62 MHz

A small osc.2nd harmonic


Transistors

• Then observe the mixer output at the test point:

• The mixer output is working as expected

– With MHz high-side tuning of the LO needs to beat kHz to give difference at 455kHz

– Note the tuning slug on the 42IF101 (yellow) is adjustable,so the IF center frequency may not be exactly 455 kHz

Time Base 100s/divOscillator test point

Mixer/1st IF test point

1.46 MHz

454.8 kHzLO leakageat Mixer/IFtest point

IF Signal

LO Signal

Frequency translated AMsignal (1M to ~455kHz)

Difference freq.signal

fc 1.0=fc fIF+ 1455=



– For this test the AM signal amplitude is 200mv and themodulation index is just 40%

• In a complete superheterodyne design one or two (better)additional IF amplification and filtering stages are required

• The output of the final IF stage will then have signal ampli-tude large enough to turn on an envelope detection diode

Integrated Devices

• The invention of the transistor in 1947 eventually led to theintegrated circuit in 1958

– The inventor, Jack Kilby of Texas Instruments, wasawarded the Nobel Prize in Physics for his invention in2000

• Integrated circuits for analog and digital circuit applicationsabound

• One integrated circuit is utilized in the TRF and superhetero-dyne receivers consider in this course

LM386 Headphone Amplifier

• The LM386 is a low voltage audio power amplifier originallycreated by National Semiconductor, now part of Texas Instru-ments

• The device can drive an 8 load to with greater than 500mWof audio power


Integrated Devices

• The voltage gain is adjustable from 20 (26 dB) to 200 (46dB)

• The quiescent current is around 4mA with a 6V supply andno input

• The equivalent circuit and the pin out is shown below:

• Application circuits from the data sheet

Equivalent Schematic8-Pin DIP Pinout

Added lowpassat fc ~7 kHz

3 turns on smallbead to suppress RF feedback

magnetwire



• An LTspice compatible model is available for the LM386:

– The LM386 model is composed of (1) a library subcircuitmodel LM386.lib and (2) a schematic symbollm386.asy

– Place both of the model files in the folder where your sche-matic is located

– To place the LM386 component navigate the Select Com-ponent Symbol dialog to the local folder where you haveplace the *.lib and *.asy files

Pull down and enter newfolder

With the local folderselected you can thenselect the lm386component from thelist in the lower halfof the dialog


Integrated Devices

• The pin connection 8 to 1, which contains and , setsthe amplifier gain:

– No connection or provides a gain of 20 or 26dB,while sets the gain to 200 or 46dB (at thegain is less)

• The frequency response from LTspice is:

• In the time domain, with a 10mV 1kHz sinusoid you expect avoltage gain of about , so the peak outputshould be 1.41V or 2.82V p-p:

– It is close at 1.39V peak

R1 C1

R1 R1 0 47

lower 3dB

upper 3dB

3dB BW

1043 20

141=

Input 10mV peak



– Note also that the DC current from the 9V supply isreported at ~16mA (hover over voltage source to see DC)

– Getting this circuit run off the Analog Discovery powersupply due to a current spike when power is applied; apowered USB hub might be the solution to this problem

• If the input rises to just 30mV, the output is already clipping

• A ZIP package can be found on the Web Site containing theLTspice amplifier schematic, the two files for the LM386model, and the data sheet pdf

Case Study: Tuned Radio Frequency Receiver

• To conclude this chapter I consider the tuned radio frequency(TRF) receiver and its implementation for AM broadcasting

• Building and testing this receiver on a solderless breadboardis a lab task that lies ahead for you

Input 30mV peak


Case Study: Tuned Radio Frequency Receiver

MK4841 Radio IC

• A complete TRF receiver can be assembled with very fewcomponents by starting with an integrated device such as theMK484 radio chip:

• From the above we see that this device more than covers theAM broadcast band (150 kHz to 3 MHz)

• Has very high input impedance (4 meg) so as not to lower theQ of an antenna coil – tuning capacitor tank circuit

1.Rapid Electronics Ltd, Severalls Lane, Colchester, Essex, England, CO45JS.



– Specifically this insures good frequency band selectivityfor accepting the signal of interest and rejecting nearbystrong signals

• Has very high gain (70 dB) to bring weak AM signal to anamplitude level suitable for driving a headphone amplifierwith no additional gain stages required

• What’s inside?

• A simple TRF design that uses a 1.5V battery for power:

MK4842 3

1

3.5mmHeadphoneJack

1 2 3

+

–

2.2k workswell too


References

References

[1] P. Scherez and S. Monk, Practical Electronics for Inventors,third edition, McGraw Hill, New York, 2013.

[2] A. Sedra and K. Smith, Microelectronic Circuits, 6th edi-tion, Oxford University Press, 2010.

[3] The ARRL Handbook for Radio Amateurs, ARRL – TheNational Association for Amateur Radio, 77th Edition, 2000.



Appendix A: Standard R and C Values1

Standard 5% Resistors Values

Standard 10% Capacitor Values

1.http://ecee.colorado.edu/~mcclurel/resistorsandcaps.pdf

Standard Resistor Values (±±5%)1.0 10 100 1.0K 10K 100K 1.0M1.1 11 110 1.1K 11K 110K 1.1M1.2 12 120 1.2K 12K 120K 1.2M1.3 13 130 1.3K 13K 130K 1.3M1.5 15 150 1.5K 15K 150K 1.5M1.6 16 160 1.6K 16K 160K 1.6M1.8 18 180 1.8K 18K 180K 1.8M2.0 20 200 2.0K 20K 200K 2.0M2.2 22 220 2.2K 22K 220K 2.2M2.4 24 240 2.4K 24K 240K 2.4M2.7 27 270 2.7K 27K 270K 2.7M3.0 30 300 3.0K 30K 300K 3.0M3.3 33 330 3.3K 33K 330K 3.3M3.6 36 360 3.6K 36K 360K 3.6M3.9 39 390 3.9K 39K 390K 3.9M4.3 43 430 4.3K 43K 430K 4.3M4.7 47 470 4.7K 47K 470K 4.7M5.1 51 510 5.1K 51K 510K 5.1M5.6 56 560 5.6K 56K 560K 5.6M6.2 62 620 6.2K 62K 620K 6.2M6.8 68 680 6.8K 68K 680K 6.8M7.5 75 750 7.5K 75K 750K 7.5M8.2 82 820 8.2K 82K 820K 8.2M9.1 91 910 9.1K 91K 910K 9.1M

Standard Capacitor Values (±±10%)10pF 100pF 1000pF .010μF .10μF 1.0μF 10μF12pF 120pF 1200pF .012μF .12μF 1.2μF15pF 150pF 1500pF .015μF .15μF 1.5μF18pF 180pF 1800pF .018μF .18μF 1.8μF22pF 220pF 2200pF .022μF .22μF 2.2μF 22μF27pF 270pF 2700pF .027μF .27μF 2.7μF33pF 330pF 3300pF .033μF .33μF 3.3μF 33μF39pF 390pF 3900pF .039μF .39μF 3.9μF47pF 470pF 4700pF .047μF .47μF 4.7μF 47uF56pF 560pF 5600pF .056μF .56μF 5.6μF68pF 680pF 6800pF .068μF .68μF 6.8μF82pF 820pF 8200pF .082μF .82μF 8.2μF


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Active Devices for Chapter Amplifiers and Oscillators · – Basic common emitter amplifier ... Chapter 7 • Active Devices for Amplifiers and Oscillators 7–6 ECE 3001 ... •