Single Stage Amplifiers (2)

Outline • • • Source Followers (Common Drain) Common Gate Cascode

Source Follower • • • You can’t drive a low impedance load with a common source amplifier Can be used as a buffer to drive a low impedance load Can provide voltage level shift

Input/Output Characteristic As Vin increases, ID1 increases leading to an increase of Vout.

Small Signal Gain Can you derive a Av without explicitly using the small signal model?

Source Follower Example gm=2mS gmbs=0.328 mS Res=6K Ohm Vout,pp=1.596

Vin, pp=2 Av=0.796 (from simulation) Av=0.801 (from small signal calculation)

Vin=1mV

Vin=330 mV More Distortion

Linearity as a function of Vin Amplitude

Small Signal Gain as a function of Vin g m g m depends on ID. When Vin=0, g m =0. will increase as V in increases. Thus A v will approaches 1. Notice that as you move the red vertical line horizontally, Av changes quite about. So there is quite a bit of nonlinearity.

Source Follower using an NMOS transistor as a current source If we can stabilize the current through M1, then gm1 will be stabilize against Vin. Av is less sensitive to Vin. The linearity of the circuit will improve.

Output Resistance of a Source Follower Have you seen this before?

Alternative Approach to R out

A v when R S is large If RS is sufficiently large, then the small signal gain of the amplifier can be obtained using thevenin’s equivalent circuit (see hand out)

SF with a NMOS CS Load

Design Example • SF with an NMOS Current Source Vout, pp=1.705 mV Vin,pp=2mV Av=0.8525

Av=0.857 (Analytical)

Vin=1mV

Vin=330 mV Vin=330 mV Rs=6K Vin=330 mV Rs=CS

SF with a Diode Connected PMOS Impedance into S of M2

Reduce sensitivity of Av due to g mb g m1 g mb1 is constant.

is noticeable. The NWELL of M1 is tied to its source.

Thus, body-source effect is negligible.

Extra Voltage Headroom Required by SF Without the SF, the minimum voltage of VX is VDSAT1.

If SF is used as a buffer, Vout must be greater than VDSAT3.

The minimum voltage at X is increased because we have to maintain sufficient voltage for VGS2. As a result the minimum voltage at X is raised.

Compare SF to CS

DC Level Shifter By using a SF, Vin can go to a higher voltage without driving M1 into the linear region.

Common Gate Amplifier

Direct Vs. Capacitive Coupling (Direct Coupling) (Capacitive Coupling)

Gain of CG If RS=0 and channel length modulation is ignored, A v is

Using formula from the previous slide CS followed by a CG

Input Resistance of CG Input resistance is approximately the drain resistance divided by (g m +g mb )ro Special cases: 1. RD=infinity (current source) 2. RD=0

Output Resistance of CG • Similar to CS

Cascode • Cascode=CS+CG M1 generates a small signal drain current proportional to Vin.

M2 simply routes the current to RD.

DC Bias of a cascode M1 and M2 must be kept in saturation.

V DS1 >V DSAT1 V DS2 >V DSAT2

Output Resistance of a Cascode M2 boosts the output resistance of M1 by a factor of (g m2 +g mb2 )r o2 r o1

Triple Cascode Advantage: higher output resistance Disadvantage: the minimum output voltage is equal to the sum of three overdrive voltages.

Voltage Gain

Exact Voltage Gain of Cascode with a Current Source Load

Implementation of Current Source Using Cascode

Shielding Property of a Cascode

Current Mismatch

Current Mismatch Using Cascode

Folded Cascode

Output Resistance

Choice of Device Model • • • • Break the circuit down into a number of familiar topologies Use the simplest model If the drain of a device is connected to high impedance (i.e. drain of another), then add ro to its model Introduce a more sophisticated circuit model if necessary