Hello:
When I was measuring input and output impedance (the real part with a tester) I noticed a huge change of it as I modified the circuit parameters. I was told that with a transistor follower I was going to solve that.
I found some solutions on the web but I would like to know from the experience if it s better to use FETs or BJTs and why.
Thanks!
In the emitter follower or in the source follower, the output impedance zo seen at the emitter or source is equal to 1/gm, where gm is the mutual transcondcutance of the transistor.
In case of bipolar transistor gm= IC/ Vt, where IC is the collector current and Vt is the thermal voltage. While for the MOSFET transistor gm= sq root (beta ID/2) where ID is the drain current and beta is the transconductance parameter of the transistor.
We see from these relationships that while gm is directly proportional to the current in the bipolar transistor it is only proportional to the square root of the drain current. The consequence of this is that one can achieve much lower output impedance in case of bipolar transistor at the same biasing current. Or one can achieve the same output impedance at much lower bias current.
This is the major advantage of the bipolar transistor. In addition to the nature of the dependence of gm on the bias current. It is linear for bipolar transistors and square root for the FETs.
Both amplifiers have high AC input impedance. The MOSFET draws in its gate no DC current while the bipolar transistor draws base current= IB= IC/beta where beta is the current amplification factor of the transistor.
It is believed that the bipolar transistor is superior than the MOSFETs in analog circuits.
Best wishes
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