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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 gm depends on ID. When Vin=0, gm=0. gm will increase as Vin increases. Thus Av 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 Rout Av when RS 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 gmb gm1 is constant. gmb1 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, Av is CS followed by a CG Using formula from the previous slide Input Resistance of CG Special cases: 1. RD=infinity (current source) 2. RD=0 Input resistance is approximately the drain resistance divided by (gm+gmb)ro 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. VDS1>VDSAT1 VDS2>VDSAT2 Output Resistance of a Cascode M2 boosts the output resistance of M1 by a factor of (gm2+gmb2)ro2ro1 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