## Lecture 5 ,6 & 7 :- Basics of MOS Amplifier

Common Data : VDD = 1.8V , Lmin  = 0.18µm, µ0N = 350cm2/V s, µ0P  = 100cm2/V s, |VTH | = 0.55, tox = 3.8nm, λ = 0.07V−1, χ = 0.1.

1. For a MOSFET operating in the linear (triode) region (ignore body eﬀect and channel- length modulation), find an expression for:

1. the small-signal transconductance gm
2. the small-signal drain-source output resistance rout

Comment upon your findings.  Is it desirable for an amplifier to work in the linear region? Explain.

2. Consider the simple MOS ampliﬁer shown in Figure 2. Assume λ = 0 and VDD = 1.8V.

Figure .2

Assume that the biasing for the gate side of the MOS is taken care of.

1. For maximum symmetrical swing in Vout  and for a drain current of 0.8 mA, find the required value of RD .
2. Calculate the maximum symmetrical output swing, and also the small-signal volt- age gain in this case.
3. How would the design values and the voltage gain change, if λ = 0.07 V−1?

3. Consider the circuit in Figure 3.

Figure. 3 Circuit for Q3

If ID = 0.5mA, RD = 1.5kΩ and RS = 300Ω, calculate:

1. the small-signal voltage gain
2. the input voltage which will place the NMOS on the border of linear/saturation region, assuming γ = λ = 0. What would be voltage gain under this condition?

4.Consider the circuit shown in Figure 4. Assume VDD = 1.8V . The amplifier has to be designed such that ID = 100 µA.

Figure. 4 Circuit for Q4

1. What should be the bias voltage Vb  for the required ID ?
2. What are the maximum and minimum values of Vout required to keep both M1 and M2 in saturation?
3. Calculate the small-signal voltage gain offered by this circuit .

5. Consider the circuit shown in Figure 5.

Figure. 5 Circuit for Q5

1. What are the maximum and minimum values of Vout required to keep both M1 and M2 in saturation?
2. Obtain an expression for the small-signal voltage gain offered by this circuit.

6. Consider the circuit in Figure 6. Assume VDD = 1.8 V.

Figure. 6 Circuit for Q6

1. What should be the bias voltage Vb  if this amplifier is to provide a small-signal voltage gain of 25 ?
2. What are the maximum and minimum values of Vout required to keep both M1 and M2 in saturation ?
3. How would the voltage gain change if M2 were replaced by an ideal current source that provides the same current as required in part (a)?