MOS Single Stage Amp.

from Razavi

parameters need to be considered: transconductance gm, linearity, voltage swing, (small signal) gain Av.

common-source (CS).

1. CS with Resistive load

problems:
1.1 nonlinearity (gm is input-dependent, and furthermore voltage gain)
1.2. couldn't get stable gain because of difficulty in getting precise Rd (it's difficult
to fabricate resistors with tightly-controlled values or a reasonable physical \size)

trade off:
1. with constant input, to get high gain, Rd need to increase, however, this lead
to smaller voltage swing at the output (transistor could get into triode region),
also, this leads to greater time constant (Speed) at the output.

2. CS with Diode-connected load
2.1 Diode-connected load behaves like a two-terminal resistor (proof it and calculate the
equivalent resistance using small sig. model) 1/(gm +gmb) Parall. r0 . therefore, prob. 1.2 solved.
2.2 small signal gain Av = Vout/Vin is independent of input voltage and is a weak function of device dimensions, therefore 1.1 is partially solved.

Prob,
2.3 trade-off exists: increeasing load resistance limits output voltage swing.
2.4 gm is still nonlinear though

3.CS with Current Source Load (could be a current mirror)
3.1 it bears all adavantages of CS with diode-connected load.
3.2 output impedenace and minimum required |Vds| are less strongly coupled. Meaning r_top can be increased ( by increasing L) without too much concern of lowering V_ds_bottom into triode region.

Prob.
3.3 Voltage_drain is not well defined. need a feedback loop to froced it to a known value
3.4 Increasing L necessitates increasing W (otherwise, Vgs - Vth increases, for a given Drain current), which increase capacitance at the output node.
3.5 gm is nonlineart

4. CS Stage with Source Degeneration (Rs between Source_bot and ground)
4.1 Calculate Gm (I_drain/V_IN, not gm) , and when V_in is large, Gm approach to a specific value (1/Rs). prob 3.5 solved at large input voltage.
4.2 Output impedance is even larger (prove it)
4.3 for a specific input voltage, V_out is more stable (think about why )

Prob. 3.3, 3.4 persist