Analog Circuits and Systems Prof. K Radhakrishna Rao

Lecture 19: Analog Signal Processing

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Review

�  Instrumentation Amplifier �  Integrator �  Differentiator �  Effect of Finite Gain Bandwidth Product (GB)

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Half-wave Diode Rectifier

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Half-wave Diode Rectifier – Simulation 1

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Transfer Characteristic

Half-wave Diode Rectifier – Simulation 2

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Rectifier Characteristic

Half-wave Rectifier

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Transfer Characteristic

�  R=1k; Opamp –LM741

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Precision Half-wave Rectifier

�  R=1k; Opamp –LM741

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Rectifier

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2p L

L

V RR R+

Rectifier (?) – Simulation 1

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Transfer Characteristic - RL=10k; R=1k; Opamp-LM741

Rectifier – Simulation 2

�  RL=0.5k; R=1k; Opamp-LM741

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Rectifier – Simulation 3

�  RL=10k; R=1k; Opamp-LM741

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Rectifier – Simulation 4

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�  Simulate with R=1k; RL=0.5k

Precision Half-wave Rectifier

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Precision Half-wave Rectifier (Transfer Characteristic)

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R=RL=1k; Opamp –LM741

Precision Half-wave Rectifier (Simulation)

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R=RL=1k; Opamp –LM741

Precision Full-wave Rectifier

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Precision Full-wave Rectifier – Simulation

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Logarithmic Amplifier (Data compressor)

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d

T

VVi

d s

iO d T

S

Vi IR

VV V V lnI R

= = ε

= − = −

S T

S

I VI

°°

where is reverse saturation current and is kT/q (26 mV at 300 K)

As doubles every 10 K rise in temperature

Temperature compensation is required.

Logarithmic Amplifier - Simulation

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1volt/decade

Transistor based Log Amplifier

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0

0

io T

E

i

VV V lnRI

V

= −

>

Log Amplifier (Transfer Characteristic)

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Log Amplifier (Transient Response)

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Square wave (0.1 to 1 V) input ; Time Period - 1msec

Transistor based Log Amplifier

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0

0

io T

E

i

VV V lnRI

V

= −

>

IEO is reverse saturation current of the emitter-base junction of the transistor The loop gain is enhanced by the transistor amplifier within the loop. In order to make Q = 1 for the loop suitable value of the capacitor CC is inserted

Log Amplifier (Transient Response)

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Modified Log Amplifier

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2

1 0

1 io T

E

R VV V lnR I R

⎛ ⎞= − +⎜ ⎟

⎝ ⎠

Temperature Compensated Log-amplifier

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2

1

1 io T

R

R VV V lnR V

⎛ ⎞= − +⎜ ⎟

⎝ ⎠

Commercial Log Amplifier (LOG2112)

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Parameters of LOG2112 Scale Factor (V/decade) 0.5    

Input Current Range (Min) (nA) 1    

Input Current Range (Max) (mA) 3.5    

Conformity Error (initial 5 decades) (Max) (%) 0.2    

Conformity Error (initial 5 decades) (typ. over temp) (%/degree C) 0.0001    

Offset Voltage (Input Amplifiers) (Max) (mV) 1.5    

Vs (+/-) (Min) (V) 4.5    

Vs (+/-) (Max) (V) 18    

Iq (Max) (mA) 1.75    

Reference Type Internal    

Auxiliary Op Amps 1    

Full-Scale Output Voltage (Min) (+/- V) -3.8    

Full-Scale Output Voltage (Max) (+/- V) 3.5    

Operating Temperature Range (C) -5 to 75    

Approx. Price (US$) 7.90 | 1ku   

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Exponential amplifier (Data expander)

�  Diode Version

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0

i

T

VV

o s

i

V I RV

⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠= − ∈

>

Exponential Amplifier (Data expander) (contd.,)

�  Transistor Version

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0

0

i

T

VV

o E

i

V RI eV

⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠= −

>

Exponential amplifier (Transfer Characteristic)

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Compensated exponential amplifier

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1

1 2

0

i

T

R VR R V

o ref

ref

V V eV

⎛ ⎞⎜ ⎟⎜ ⎟+⎝ ⎠=

>

Log-Antilog Multiplier

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Applications

�  LOG, LOG Ratio: Communication, Analytical, Medical, Industrial, Test, General Instrumentation

�  Opto-coupler isolation amplifier �  Analog signal compression in front of A/D converter �  Absorbance Measurement �  Optical density measurement

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Opto-coupler isolation amplifier

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Conclusion

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Conclusion

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