SOLUTION

The input and output impedance of transistors depends on quite a lot on the technology of the transistor and on the way it is used in a circuit.

A bipolar transistor used as a common collector amplifier will have a high input impedance and a low output impedance.

The same transistor used as a common base amplifier will have a low input impedance and a high output impedance.

We typically don’t like low input impedance because we would like to control things using very little current. A low input impedance draws current. Also, we typically like low output impedance when the transistor is on and a high output resistance when it is off.

Advantages of bipolar junction transistor (BJT) are as given below,

1. The bipolar junction transistor (BJT) has a large gain bandwidth.
2. The BJT shows better performance at high frequency.
3. The large value of current gain, β
4. The large value of transfer conductance g_m
5. The BJT has a better voltage gain.
6. The BJT can be operated in low or high-power applications.
7. The BJT has a high current density.
8. There is a low forward voltage drop.
9. BJTs have  a very predictable turn-on voltage, i.e. typically 0.7V Vbe
10. A BJT needs a small amount of current to switch on the transistor.
11. BJT amplifier stages are much more linear than MOSFET amplifier stages, as the gain doesn’t depend on the bias voltage.  This gives better fidelity.
12. BJT’s are capable of handling higher output currents for signal outputs and can have a lower output impedance.  In amplifiers intended to drive a low input impedance load or deliver significant amounts of power, this is a huge advantage.  Many of the highest quality op-amps are made with a BiCMOS process using a BJT pair for the output buffer stage.

There are some disadvantages of bipolar junction transistor (BJT) are as given below,

1. The bipolar junction transistor (BJT) more noise produced.
2. Special isolation regions are required between individual devices in an
3. More number of diffusion steps are required for its manufacturing
4.  Larger turn-off time
5. Operation of the BJT relies on the transport of minority carriers and that is significantly affected by incident electromagnetic radiation. (Hence, the existence of photo-diodes and photo-transistors, etc.)
6. BJT has low thermal stability.
7. BJT’s are current-operated devices rather than voltage-operated.  Most of the time this means higher power consumption.
8. BJT has a positive temperature coefficient at a high current level. It means in BJT collector current increases with the increase in temperature. This characteristic leads the BJT to thermal breakdown.
9. The switching frequency of BJT is low.
10. It has a very complex base control. So it may lead to confusion and requires skillful handling.