Arrange the below referred amplifiers in the decreasing order of the input impedance : (a) Common Base Transistor Amplifier (b) Common Emitter Transistor Amplifier (c) Common Collector Transistor Amplifier (d) Operational Amplifiers Options :
(d), (c), (b), (a)
Work out the three BJT configurations first, then place the op-amp.
Common base — the lowest. The signal is driven straight into the emitter, so the source faces the forward-biased emitter junction directly:
\(Z_{in(CB)}\approx r_{e}=\dfrac{26\ \text{mV}}{I_{E}}\)
which for a 1 mA bias is only about 26 Ω. There is no \(\beta\) multiplication because the input current is the emitter current.
Common emitter — moderate. The signal enters the base, so the emitter resistance is seen through the current gain:
\(Z_{in(CE)}\approx\beta r_{e}\)
typically 1 kΩ to a few kΩ.
Common collector (emitter follower) — high. The emitter resistor is also reflected into the base, and it is far larger than \(r_{e}\):
\(Z_{in(CC)}\approx\beta\left(r_{e}+R_{E}\right)\)
typically hundreds of kΩ. This bootstrapping of the load into the input is exactly why the follower is the standard buffer stage.
Operational amplifier — highest of all. The op-amp input is a differential pair, usually itself a high-impedance stage, and negative feedback multiplies the open-loop input resistance by the loop gain. A bipolar-input op-amp offers megohms; a FET or CMOS input offers \(10^{12}\ \Omega\) and picoamp bias currents.
| Amplifier | Typical Zin | Reason |
|---|---|---|
| Op-amp | MΩ to TΩ | Differential stage plus feedback |
| Common collector | Hundreds of kΩ | \(\beta(r_{e}+R_{E})\) |
| Common emitter | ~1–5 kΩ | \(\beta r_{e}\) |
| Common base | Tens of Ω | \(r_{e}\) only |
The decreasing order is (d), (c), (b), (a) — option 3.
The output impedances run the other way, which is why each configuration has its niche: the emitter follower has a very low output impedance and is used to drive heavy loads, while the common base, with its low input and high output impedance, suits high-frequency work and is the standard input stage of a cascode.
Hence, the correct sequence is (d), (c), (b), (a).
Similar Questions
The C.E. configuration is normally preferred because it provides :
i. voltage gain
ii. current gain
iii. power gain
iv. stabilityWhich is correct ?
Match the following :
List – I List – II a. CE-amplifier i. Low bandwidth high input impedance amplifier b. CB-amplifier ii. Audio frequency amplifier c. JFET amplifier iii. Radio frequency amplifier d. CC-amplifier iv. Buffer amplifier Codes :
Read the statements regarding transistor.
A. The dopping level of emitter region is more than base region but less than collector region.
B. The CB configuration is a good current amplifier circuit configuration
C. The phase difference between I/P and O/P waveforms of a CB configuration amplifying circuit is 0°.
D. CC configuration transistor amplifier has higher value of I/P resistance and lower values of O/P resistance.
Choose the correct answer from the options given below:
Following devices are given :
(a) transistor in CE stage
(b) transistor in CB stage
(c) transistor in CC stage
(d) Op-Amp
The arrangement of their current gain in ascending order is given by
Important Questions from Configuration of BJT
For the CE-transistor amplifier, the audio signal voltage across the collected resistance of 3 kΩ is 3V. Assume the current amplification factor of the transistor is 50, and find the input voltage and base current, if the resistance is 1 k Ω ?
Which of the following is NOT true for a common collector transistor?
The voltage gain of a Common emitter amplifier ______, as the load resistance is increased
Find the value of β for a BJT having α = 0.99.
The current gain of amplifier stage is lowest in
