11. How to choose an op-amp when working with high input source resistance?
A. Op-amp with low bias current
B. Op-amp with higher slew rate
C. Buffer or voltage follower
D. All of the mentioned
Answer: D
When the op-amp is driven by a high input source resistance, the output and input voltage will not be equal due to an error at the input. A remedy to this problem is an op-amp with a low input bias current and a high slew rate should be chosen as a voltage follower.
12. What must be done to block the ac input voltage riding on a dc level?
A. Use RC network
B. Use coupling capacitor
C. Use resistive transducer
D. None of the mentioned
Answer: B
To block the dc level a coupling capacitor must be used in series with the input of the voltage follower.
13. To get higher input resistance in AC coupled voltage follower,
A. The output resistance is bootstrapped
B. The input resistance is bootstrapped
C. The bias resistance is bootstrapped
D. The feedback resistance is bootstrapped
Answer: C
Bias resistor connected to the ground to provide a path in an AC coupled voltage follower, drastically reducing the input resistance of the circuit. Therefore, to get high input resistance, the bias resistance is bootstrapped.
14. Voltage follower circuits are used in
A. Active filter
B. All of the mentioned
C. Sample and hold circuit
D. Bridge circuit with transducer
Answer: B
Voltage followers are useful for all the above-mentioned applications because they involve working with high-input source resistance.
15. Compute the reference voltage for a fundamental log-amp, if its internal resistance=5MΩ.
A. 0.5µv
B. 0.05µv
C. 5µv
D. None of the mentioned
Answer: A
The reference voltage, Vref = R1 × Is
Where, Is~10-13A (for an emitter saturation current).
∴ Vref = 10-13 × 5MΩ
= 5×10-7 = 0.5µv.
16. Which of the following functions does the antilog computation required to perform continuously with log-amps?
A. In(x)
B. log(x)
C. Sinh(x)
D. All of the mentioned
Answer: D
Log-amp can easily perform function such as In(x), Log(x), Sinh(x) to have direct dB display on digital voltmeter and spectrum analyser.
17. Find the output voltage of the log-amplifier
A. VO = -(kT)×ln(Vi/Vref)
B. VO = -(kT/q)×ln(Vi/Vref)
C. VO = -(kT/q)×ln(Vref/Vi)
D. VO = (kT/q)×ln(Vi/Vref)
Answer: B
the output voltage is proportional to the logarithm of the input voltage.
VO =-(kT/q)×ln(Vi / Vref).
18. How to provide saturation current and temperature compensation in log-amp?
A. Applying reference voltage alone to two different log-amps
B. Applying input and reference voltage to same log-amps
C. Applying input and reference voltage to separate log-amps
D. None of the mentioned
Answer: C
The emitter saturation current varies from transistor to transistor with temperature. Therefore, the input and reference voltage are applied to separate log-amps and two transistors are integrated close together in the same silicon wafer. This provides a close match of the saturation currents and ensures good thermal tracking.
19. The input voltage, 6v, and reference voltage, 4 v are applied to a log-amp with saturation current and temperature compensation. Find the output voltage of the log-amp?
A. 6.314(kT/q)v
B. 0.597(kT/q)v
C. 0.405(kT/q)v
D. 1.214(kT/q)v
Answer: C
The output voltage of saturation current and temperature compensation log-amp
VO = (kT/q)×ln(Vi / Vref)
=(kT/q)×ln(6v/4v)
=(kT/q)×ln(1.5)
VO = 0.405(kT/q)v.
20. Determine the output voltage for the given circuit
A. VO = Vref/(10-k’vi)
B. VO = Vref+(10-k’vi)
C. VO = Vref×(10-k’vi)
D. VO = Vref-(10-k’vi)
Answer: C
The output voltage of an antilog amp is given as
VO = Vref (10-k’vi)
Where k’ = 0.4343 (q/kt)×[(RTC/ (R2 +RTC)].
21. Calculate the base voltage of the Q2 transistor in the log-amp using two op-amps?
