Cascode Amplifier: clarifications about output resistance and working point of transistors

Question

let's consider this cascode amplifier (here the reference):

I have the following questions:

1) Is it good as a voltage amplifier or as a transconductance amplifier?

I do not understand it because: - voltage gain is evaluated, so it seems that it is used as a voltage amplifier - when the output resistance is evaluated, we see that it is very high, that is not good for a voltage amplifier. Moreover, I have read in my notes that cascode is used sometimes in order to increase the output resistance in order to have a better behaviour as a current source. But if it is true, I do not understand how can it be a current source, since the load will receive a current which is not equal to the drain current, but to I0 - drain current (except the case in which the load is a short).

2) Do we prefer designing Q1 and Q2 to work in saturation or in triode? And why?

Answer 1

Voltage Amplifier :

The gm of the structure is still the gm of the bottom transistor while the output impedance is much higher than a CS amplifier.This helps boost the small signal gain of the device, provided the Io in the picture is also implemented as cascode, otherwise the gain will be limited by the output impedance of the current source Io. However cascoding comes at the expense of reduced voltage headroom at the output.

Transconductance Amplifier:

The output of interest is the small signal output current , the gm of the device is unchanged, however the current into the load will be a higher fraction of gm.vi due to its increased output impedance, more of the gm.vi current will into the load.

Current Sources :

The idea here is that an ideal current source has infinite output impedance , that is the current it sinks or sources is independent of the voltage across it.

Now when you implement the Io in the above image as a cascode , any small change at the output voltage should not change the Io value as it will mess up the biasing of the circuitry. If Io was just one PMOS , any change at the output would have directly changed its Vds and hence the mirroring of the current Io from the reference. Cascoding helps in reducing the change in Vds by a factor of gm.ro which helps it mirror better and be better current source.

I have assumed that you know how Io in the figure will be implemented, if not please go through slides of Allen Philip online.

Answer 2

It is a good transconductance amplifier because it has high input impedance, good for voltage input, and high output impedance good for current output. But you are misinterpreting it as a constant current source, instead it is a voltage controlled current source whose transconductance is gm (of the lower transistor). In other words, transconductance amplifier is good for driving a current signal into a load, Rl in your case.

Both the transistors need to be in saturation since the output impedance is lowered in the triode region, hampering the performance of transconductance stage.