Gain vs Output Impedence

I am currently trying to improve my understanding of analogue IC design and my textbook kind of brushes over this problem.

It is made quite clear that a low output impedance is desired so that the output voltage and current are mostly spread across the load impedance. However, in design (specifically in small signal analysis) a very common way of improving the gain is to increase the output resistance (since transistors amplify by means of the output current).

I understand that both can be achieved in multi-stage devices ex. with a buffer amplifier stage at the output.

My question is: how do I reconcile these design goals, particularly in single transistor circuits?

Cheers

In a single amplifier stage you can't have low output impedance and high gain at the same time. The active device (bipolar or MOSFET) is voltage controlled and generates a current. In order to get a voltage again you need a resistor. Therefore you have to make a tradeoff between gain and low output resistance.

The output resistance is only of concern when resistive loads are driven. In analog IC design the loads are often capacitive and OTAs (amplifiers with a high output resistance) are perfectly fine for such a task.

If it is needed to drive a resistor either a buffer stage (emitter/source follower) or feedback is required to achieve that goal, which usually requires a multi-stage design.

Voltage gain can be increased (for BJT) by increasing the collector current. as gm = Ic/Vt. For a MOSFET gm increases with the square root of the drain current.

Of course that has the effect of increasing the power dissipation and in the case of the BJT it lowers the input impedance so that current gain stays the same.

Output impedance depends on if the gain is for extra current, or for extra voltage or voltage swing. Current gain is normally taken off the emitter (of a transistor )and implies there is no voltage gain, but the drive impedance is lowered, possibly a great deal.

This is sometimes called a 'Buffer', because it isolated the previous stage from the next stage, which could be a speaker, if the circuit was an amplifier. Voltage gain is normally taken off the collector (of a transistor ) and implies there is no current gain-or very little at most.

This is why amplifiers of many types, including audio and 'op-amps' have the signal voltage amplified to the desired level first, then use buffers so the drive voltage can drive low or very low impedance loads.