RF POWER AMPLFIERS (3)

Mihai Albulet

윤석현

Broadband impedance matching 이 최선이라도 , 종래의 transformer 는 거의 RF power amplifier 에서 유용하지 않다 .

Basic Limitation of the convention Transformer.

2.7 Broadband Matching Circuit

Basic Limitation of the conven-tion Transformer

Primary 와 secondary coil 에 걸린 voltage 는 coil 의 감긴 수 (turn) 에 비례 .

Coli 에 흐르는 전류는 감긴 수에 반 비례 .

Basic Limitation of the conven-tion Transformer

2

1

2

2

2

1

n

nn

RnRn

nR LL

The following presentation of these frequency limitation is base on the lumped equivalent circuit of the wideband transformer shown in figure 2-73

Basic Limitation of the conven-tion Transformer

Basic Limitation of the conven-tion Transformer Is the load resistance referred to primary winding

r1 is the series resistance of the primary winding

is the series resistance of the secondary winding re-ferred to the primary

Rc models the power loss in the magnetic core

LL RnR 2'

2' 2rnr

Fig 2-73 을 low, middle, high 로 주파수 범위 구분하여 equiva-lent 회로 만든다 .

Mid band 에서 리액턴스 값 무시 . Rc may be ignored, although it should be included in the

equivalent circuit if the power level is high and/or the power loss in the magnetic core is significant

Basic Limitation of the conven-tion Transformer

Lower frequency

The capacitance and the leakage inductance are negligibleShunt inductance Lp become important Shunt inductance Lp 가 low frequency 제한의 주 원인 .

Basic Limitation of the conven-tion Transformer

High frequency

The Capacitance and the leakage inductance become im-portant .

Lp and Rc negligible, and r’1 and r’2 are usually ignored

Basic Limitation of the conven-tion Transformer

In the transmission-line transformer the coil are arranged so that interwinding capacitance combine with the induc-tance to form a transmission line.

As a result, the high-frequency response is limited by the parasitics which have not been absorbed into the character-istic impedance of the transmission line or by the deviation of the characteristic impedance from its optimum values

Transmission-Line Transformer

The bifilar coil is usually constructed by coiling a transmis-sion line around a ferrite core, or by threading the line through ferrite beads

Ganella’s Transmission-Line Transformer

Bifilar coil 에 흐르는 전류를 odd-mode (io) 와 even mode(ie) 로 나눌 수 있다 .

the odd-mode current 는 무시할만한 external magnetic field 를 발생 .

그 결과 , adjacent turns 사이에서 magnetic coupling 이 발생하지 않고 , 같은 길이의 전송선과 같아 진다 .

The even-mode current 는 in-phase equal magnetic fields 발생 , 그 결과 인덕턴스가 높을 때 standard coil L 과 같아 진다 .

Inductance L 이 상당히 높으면 , even-mode 전류는 무시 할 만큼 작아지고 , odd-mode 로만 작용 .

Ganella’s Transmission-Line Transformer

(a) The bifilar coil behave as a delay line.

Ganella’s Transmission-Line Transformer

(b) Fig 2-78 은 phase inverter. L 의 리액턴스가 RL 보다 커지면 단지 odd-mode 로 전류 흐른다 . V1 과 V2 는 out-of-phase.

(c) Fig 2-78 은 balun (balanced-to-unbalanced) configurationRL 의 중심이 left off 이면 두 windings 의 전류는 equal 이고 op-

posite 하다 .

The reactance of RL is connected to ground, as shown in fig 2-78( c) ,the reactance of L must be far greater than RL to assure that the even-mode currents are negligible and the load is balanced to ground

Ganella’s Transmission-Line Transformer

Fig 2-79

Ganella’s Transmission-Line Transformer

The two bifilar coils are in parallel at the low-impedance side(signal source) and in series at the high-impedance side(the load)Characteristic impedance Z0=RL/2 , R=RL/2 (Fig 2-80)

Ganella’s Transmission-Line Transformer

Characteristic impedance Z0=nRL , (Fig 2-81)

Ganella’s Transmission-Line Transformer

LRnR 2

Guanella’s transmission-line transformer 제한

1. transmission-line 의 characteristic impedance 에 의해 흡수되지 않은 parasitic 성분 .

2. 주파수에 따른 Z0 의 변화 .

3. 증가하는 주파수에 따른 transmission-line 에서 power loss 증가 .

Ganella’s Transmission-Line Transformer

- Low-frequency model of Guanella’s 1:4 Transmission-Line Transformer -

Ganella’s Transmission-Line Transformer

1. the circuit is used as a balun with floating load, i.e , Only terminal (1,5) is ground. The low frequency response is

given by the magnetizing inductance (that shunt the signal source), comprised of winding 3-4 in series with winding 6-5.

a. If two transmission lines are coiled on separate core, the magnetic inductance is the sum of the two inductance (3-4 and 6-5).

b. If two transmission lines are coiled on the same core, the magnetic coupling between them must be considered.

Low-frequency model of Guanella’s 1:4 Transmission-Line Trans-former

2. The circuit is used as an unun transformer, with terminal (1,5)

And 2 connected to ground. Consequently, winding 1-2 is shorted, as is winding 3-4.

a. If two core are used, winding 5-6 and 7-8 are not affected by shorted winding 1-2 and 3-4. 저주파 응답은 winding 5-6 and 7-8 에 의해 제공됨 . 둘 사이에 magnetic coupling 발생 .

b. If one core is used, winding 5-6 and 7-8 are also shorted, resulting in a very poor low-frequency response.

If a good low-frequency unun transformer response is needed, it is best

Low-frequency model of Guanella’s 1:4 Transmission-Line Transformer

Ruthroff’s Transformer-line TransformerRuthroff’s Transformer-line Transformer mainly sums a direct

voltage (or current) with a delay voltage (or current ) , which transverses a transmission lines.

Low-frequency model of Guanella’s 1:4 Transmission-Line Transformer

a. Winding 은 충분한 길이의 리액턴스를 가진다 . 그래서 단지 odd-mode current 가 bifilar coil 에 흐른다 .

b. 이 회로의 주요 목적은 signal source Vs 로부터 Load RL 까지 wide frequency band 내에서 power 의 maximum 전송을 얻는 것이다 .

Ruthroff’s Transformer-line Transformer

The low-frequency model of the Ruthoff 1:4 transformer is shown in Fig 2-87

이 회로는 1:4 autotransformer 와 매우 유사함 .

Ruthroff’s Transformer-line Transformer

Fig 2-87 (a) 의 더 간단한 형태가 Fig 2-87 (b)

• Core magnetizing inductance LM 의하여 shunt 된 ideal transformer 로 대체됨 .

• Low frequency 를 향상시키기 위한 방법은 LM 증가 . LM 의 증가 방법a. Bifilar coil 의 turn 수 증가 .b. Magnetic coil 의 permeability 증가c. Coil 의 effective cross-sectional area 증가d. 코일에서 average magnetic path length 감소 a, c 는 전송선 길이의 증가에 의해 발생 .

Ruthroff’s Transformer-line Transformer

A general synthesis procedure for an arbitrary im-pedance ratio (where m and n are integers)

이 기술은 Guanella’s circuit 과 비교해 더 복잡하고 확장적 기술 .

Other types of Transmission-line Transformers

22 : nm

Fig 2-90 은 3:5 voltage ratio transformer ( 즉 9:25 imped-ance ratio)

Other types of Transmission-line Transformers

Fig 2-91 은 bottom transformer line 이 1:4 Ruthroff unun 로 연결 . Top transformer line 은 Guanella 1:1 balun 로 연결 .

Other types of Transmission-line Transformers

Other types of Transmission-line Transformers

Broadband RF power amplifiers often create two additional problem

a. The overall gain must be relatively flat over the operating frequency range fmin …. fmax.

b. The input VSWR of the amplifier must be kept within a specified range (for example, 2:1 or lower) over the oper-ating frequency range.

두 가지 문제가 각각 따로 논의 되었더라도 각각의 문제는 서로 완전히 분리 할 수 없다 .

두 가지 문제 모두 amplifier input 과 feedback path 의 다양한 R,C,L 성분에 의해 해결 된다 .

2.8 Gain Leveling and VSWR Correction

Broadband amplifier 에서 gain disparity 는 10~15 dB 보다 훨씬 크다 . Gain leveling 의 가능한 해결책은 frequency-dependent shunt feedback

을 사용 .

Cf is a DC-blocking capacitor.

Lf tend to increase The impedance ofthe feedback path withThe operating frequency,Preserving amplifier gain atHigher frequenciesAt lower frequencies, the feedback become stronger, decreasing am-

plifiers gain as well as the input impedance Zin

Gain Leveling and VSWR Cor-rection

Fig 2-96 은 degenerative feedback

CE reduce the gain at low frequencies이미터 회로에서 원하지 않는 parasitic capacitance 를 제거 하기

어려움 .

Gain Leveling and VSWR Cor-rection

Gain leveling can also be accomplished using RLC network, asFig 2-97.

At low frequencies, L has a low reactance while C has a high reac-tance. 그 결과 base-emitter junction 의 power 양 감소 ->Overall gain 감소 .

At high frequencies, L 과 C 는 power gain 을 보존하는 R1 , R2 영향을 제거함 . 그래서 input impedance 는 주파수 또한 변화시킴 .

Gain Leveling and VSWR Cor-rection

Four-reactance networks can also be used for broadband matching (see Fig 2-99).

A four-reactance networks is able to transform load imped-ance R into Rin, at two frequencies.

Gain Leveling and VSWR Cor-rection

Three basic procedures can be used to obtain a high-power AM signal: 1. base bias modulation 2. AM signal amplifica-tion 3.collector modulation

1. base bias modulationLow frequency modulating signal 은 class c 의 base bias 를

control 하기 위해 사용됨 . Base bias voltage 가 conduction angle 에 영향 . 그래서 AM signal 로 output signal 발생 .

단점a. 낮은 efficiency b. modulation 특성의 비선형성c. signal level 에 따라 달라지는 base-emitter capacitance 값의

변화 때문에 amplifier 가 잘못된 동작상태에서 modulation sig-nal 제공 .

2.9 Amplitude Modulation

2. Am signal amplificationSSB transmission 널리 사용 . SSB signal 은 low power level 에서 쉽게 동작하고 바람직한 power

level 에서 선형적 증폭 . Class AB push pull amplifier 로 동작 가능 .

3. Collector modulationSolid state RF amplifier 에서 사용된 amplitude modulation 의

주요 형태 .AM double sideband (DSB) signal 얻을 수 있다 .In an envelope elimination and restoration (EER) system, col-

lector modulation can be used to generate any type of AM signal

Amplitude Modulation

The basic circuit of collector-modulated RF amplifier

1. 이 회로의 중요한 이점은 RF amplifier 가 high efficiency 로 동작 .

2. 주요 단점은 modulating signal 이 high power level 에서 증폭 되야 함 .

As a result, the efficiency of this amplifier is also important for the overall efficiency of transmitter.

The modulation transformer must be able to low frequency, high power signals with low distortion and high efficiency.

The basic circuit of collector-modulated RF amplifier

Amplitude Modulation

Frequency multipliers 는 master oscillator 의 주파수를 multi-plier 하거나 modulation index 의 증가시키기 위해 사용된다 .

Class C frequency multiplier 는 Class C power amplifier 와 같은 schematic 을 가짐 .

The only difference is that the collector resonant circuit is tuned to the desired harmonics, suppressing all other har-monics

Class C Frequency Multi-pliers

The variation of the maximum collector efficiency with the conduction angle , for a Class C amplifier (n=1), a double (n=2) , and a tripler (n=3) is shown in Fig 2-102 .

Collector efficiency decreases as The multiplying orderN increases

Class C Frequency Multi-pliers

maxc

A problem common to any type of frequency multiplier is the output voltage damping (Fig 2-104)

This effect become stronger as multiplication factor n in-crease and output Q decreases.

Class C Frequency Multi-pliers

RF power amplifier instability manifests itself in spurious oscilla-tions.

These oscillation can occur at frequencies related or unrelated to the operating frequency (for example, harmonics or subharmon-ics ).

• Spurious oscillation can occur under a particular set operating condition, such as frequency, bias, signal level, load impedance, and temperature.

• Some different oscillation mechanisms may coexist in the same circuit.

• It is very difficult to construct a model of the RF power amplifier.• Achieving stability may require some sacrifice of gain, selectiv-

ity, or overall efficiency.

2.11 Stability of RF power Ampli-fiers

Positive linear feedback can be caused by transistor inter-electrode capacitances and load inductance or by capacitive or magnetic coupling between the input and output circuit.

RF power amplifier is always a nonlinear system .Even Class A amplifier is nonlinear system.

In these case, the quasi-linear approach must used, and the non linear element of the circuit must be replaced by their linear equivalent.

The main problem is active device.• The transistor parameters change during the RF cycle. The conditions required to start or sustain oscillation could be

fulfilled only for certain values of DC-supply voltage and RF signal level.

Linear Feedback

In most RF power amplifier it is almost impossible to separate linear feedback from feedback mechanisms.

Consequently, it is desirable to avoid or to reduce linear feedback as much as possible.

Linear Feedback

RF power transistor often dissipate a signifi-cant amount of power that is converted to heat.

The operating temperature of the junction is determined by the dissipated power, and the thermal resistance of the transistor, heatsink, and ,mounting hardware.

Thermal Feedback

Experience shows that if the RF chokes and damping ef-fects are improperly chosen, self-oscillation may appear in RF power amplifiers.

Am explanation of this behavior is based on nonlinear oscil-lation mechanism in the transistor bias network.

A possible positive feedback path may be established in the circuit. The oscillation appears at a frequency lower than the intended operating frequency.

To avoid this type of oscillation, it is best to provide resis-tive damping in the base bias circuit (Fig 20-106)

Oscillation Caused by the Transis-tor Bias Network.

Oscillation Caused by the Transis-tor Bias Network.

The theory of circuit with variable parameter show that os-cillations can be generated by periodically varying the pa-rameter (capacitance or inductance) of one of the energy-storing elements of a tuned circuit.

Parametric oscillation usually occurs at a subharmonic of the circuit operating frequency (f/k, where f is the operating frequency and k is an integer), although it is no necessary that the oscillation frequency is an integer submultiple of f.

Parametric Oscillation

Such oscillation may occur if the base-collector junction is forced into forward conduction part of RF cycle.

The direct connect between the input and output tuned circuit may cause oscillation.

Load current Drawn through Base-collector junction in Forward Coduction

Oscillation Caused by the Base Stored Charge may occur in amplifier in which the transistor saturates during the RF cy-cle

(for instance, overdiven Class C, mixed-mode, or switching-mode amplifier)

The transistor remain saturated during the transistor stor-age time, which depends on the base and collector current

Oscillation Caused by the Base Stored Charge

Q&A