EE05 Fall 205 Microelectronic Devices and Circuits Prof. Ming C. Wu wu@eecs.berkeley.edu 5 Sutardja Dai Hall (SDH 2- MOSFET Basic Single-Transistor Amplifier Configurations BJT 2-2
Two-Port Model of Amplifiers + 0 In addition to gain, + it's important to have proper input and output resistances. Procedure to find : Ground input, remove Apply a test current sourceat output (conceptually, not experimentally, find voltage at output terminal. 2-3 0 v x i x Common-Source (CS Amplifier Previously, using MOSFET equation (on p. -4, we derived analytically : A V k n V OV (The process is combersome, tedious, and will go quickly out of hand with more complex circuits Using hybrid-pi model, it is almost trivial to solve: g m With load resistance, : Since is in paralle with g m ( 2-4 0 2
Common-Emitter (CE Amplifier Again, using hybrid-pi model, it can be easily solved: g m C With load resistance, : Since is in paralle with g m ( C + r π r π C r π 2-5 T-Equivalent Circuit Model for MOSFET (without r o No change in circuit since the additional dependent current source has the same current Same current with the dependent current source replaced by equivalent resistance No change in circuit since no current will flow through the new connected path 2-6 3
CS with Source esistance (Source Degeneration You can analyze the circuit using hybrid-pi model. (Try it! However, whenever there is a resistor connected to source, it is much easier to use the "T-model" i " % i$ + S ' # g m & D g m + + g m S S g m ( Grain reduced by + g m S compared with standard CS amplifier 2-7 CS with Source esistance (Source Degeneration Note : D Total resistance in Drain + Total resistance in Source S g m S provides negative feedback, which ( stablize drain current (2 increse linearity by keeping v gs small g v gs v m i v i + + g m S S g m (3 Increase usable bandwidth 2-8 4
T-Model for BJT Hybrid-pi Model T-Model without r 0 T-Model with r 0 2-9 CE with Emitter esistance (Emitter Degeneration i c C ( i e + e C i c α g m C + e i e + g m e where α i c β (usually α i e β + Note: is boosted signficantly: ( i e + e # & ( β + % + e ( i b $ g m ' r π + ( β + e C 2-0 α g # m C & in + g m % e $ + ( in ' 5
CE with Emitter esistance (Emitter Degeneration Effect of e (called "Emitter Degeneration": ( increase input resistance by (+ g m e (2 reduce voltage gain by (+ g m e (3 reduce v be by (+ g m e lower nonlinear distortion (4 voltage gain less dependent on β (5 improved high frequency response 2- Common-Gate (CG Amplifier CG Amplifier CG Amplifier with T-Model connected to source Use the "T-model" i i( g m n / g sig v m sig + g m 2-2 Low input resistance 6
Common-Base (CB Amplifier connected to Emitter Use the "T-model" i C i( C g m C Low input resistance n / g sig v m sig + g m C C C 2-3 Unity-Gain Buffer Voltage Amplifier Driving low impedance load directly: Driving low impedance load with unity-gain buffer: 2-4 7
Common-Drain (CD Amplifier (Source Follower Source Follower Source Follower with T-Model connected to source Use the "T-model" when >> 2-5 Low output resistance Common-Collector (CC Amplifier (Emitter Follower Emitter Follower Emitter Follower with T-Model connected to Emitter Use the "T-model" when >> (β +( 2-6 (β +( + (β +( High input resistance Low output resistance 8
Input and Output esistance of Emitter Follower Intuition: esistance on emitter side is multiplied by (+β when seen from base (input side High input resistance esistance on base side is divided by (+β when seen from emitter side Low output resistance 2-7 Comparison of Different Amplifier Configurations MOS has higher input ( impedance BJT has higher gain (g m IC uses MOS, discrete circuits favors BJT CS / CE provides the bulk of the gain CD / CC used as voltage buffer in output stage 2-8 9
Summary of MOSFET Amplifiers 2-9 Summary of BJT Amplifiers 2-20 0