Analysis of Single Stage Amplifiers

Small Signal Models Small-signal models for the MOSFET: (a) neglecting the dependence of iD on vDS in saturation (channel-length modulation effect); and (b) including the effect of channel-length modulation modeled by output resistance ro = |VA|/ID.

Small Signal T Models The T model of the MOSFET augmented with the drain-to-source resistance ro

Single Stage Common Source Amplifier

Small Signal equivalent circuit Small Signal Model Small Signal equivalent circuit

Single Stage Cs Amplifier Input resistance (Very High input resistance ) Voltage gain (Moderately High Voltage Gain) Output resistance (Relatively High output Resistance) Voltage Gain for small signal The magnitude of the gain Av can be increased by increasing W/L Ratio if the other parameter are constant.

Common Gate Amplifier

Common Gate Amplifier

Common Gate Amplifier Input resistance Voltage gain Output resistance Non inverting amplifier Low input resistance Has nearly identical voltage gain of CS amplifier Relatively high output resistance Current follower Superior high-frequency performance

Source Follower Or Common Drain Amplifier

Small Signal Model

Source Follower Input resistance Voltage gain (Voltage Divider) Output resistance Very high input resistance Voltage gain is less than but close to unity Relatively low output resistance Voltage buffer amplifier Power amplifier

Common Source with Source Resistance

Common Source with Source Resistance

Common Source with Source Resistance Input resistance Voltage gain Output resistance Including RS results in a gain reduction by the factor (1+gmRS) RS takes the effect of negative feedback.

MOS Cascode In response to the input signal voltage vi the transistor will conduct a current gm1vgs1 and feed it to the source terminal of the common gate transistor Q2 called as cascode transistor. This configuration is called as cascode configuration. The basic idea behind the cascode amplifier is to combine the high input resistance and large Transconductance achieved in a common-source amplifier with the current-buffering property and the superior high-frequency response of the common-gate circuit.

MOS Cascode

MOS Cascode

MOS Cascode

MOS Cascode (to raise the output resistance) Numerical : It is required to design the Cascode current source to provide a current of 100µA and an output resistance of 500 KΩ. Assume the availability of a 0.18µm Technology for which and calculate L and W/L for each transistor. And the bias voltages and . Cascoding can also be employed to raise the output resistance of the current source load. Q4 is the current source transistor and Q3 is the common gate (CG) Cascode transistor. The Cascode transistor Q3 multiplies the output resistance of of Q4 to provide an output resistance for the Cascode current source.

Example

Example