This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “Log and Antilog Amplifier”.
1. Which of the following functions does the antilog computation required to perform continuously with log-amps?
d) All of the mentioned
Explanation: 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.
Explanation: Log amps are used to compress the dynamic range of a signal. The fundamental log amp circuit consists of a grounded base transistor in the feedback path.
3. 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)
Explanation: the output voltage is proportional to the logarithm of input voltage.
VO =-(kT/q)×ln(Vi / Vref).
4. 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
Explanation: 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.
5. 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?
Explanation: 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.
Explanation: The temperature dependence on the output voltage is compensated by connecting an op-amp which provide a non-inverting gain of [1+ (R2/ RTC)] at the output of the log-amp with saturation current compensation.
Now the output voltage becomes,VO = [1+ (R2/ RTC)]×[(kT/q)×ln(Vi / Vref)] Where, RTC –> temperature sensitive resistance with a positive co-efficient of temperature.
Explanation: The output voltage of an antilog amp is given as, VO = Vref (10-k’vi)
Where k’ = 0.4343 (q/kt)×[(RTC/ (R2 +RTC)].
Explanation: The base voltage of Q2 transistor, VB = [RTC / (R2 +RTC)]×(Vi) = [10kΩ/(5kΩ+10kΩ)]×5v =3.33v.
Sanfoundry Global Education & Learning Series – Linear Integrated Circuits.
To practice all areas of Linear Integrated Circuits, here is complete set of 1000+ Multiple Choice Questions and Answers.