1. Microelectronic Circuit Design, 3E McGraw-Hill Chapter 13 Small-Signal Modeling and Linear Amplification Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock

2. Microelectronic Circuit Design, 3E McGraw-Hill DC and AC Equivalents for MOSFET Amplifier dc equivalent ac equivalent Simplified ac equivalent Full circuit

3. Microelectronic Circuit Design, 3E McGraw-Hill Small-Signal Operation of Diode The slope of the diode characteristic at the Q-point is called the diode conductance and is given by: g d is small but non-zero for I D = 0 because slope of diode equation is nonzero at the origin. Diode resistance is given by:

4. Microelectronic Circuit Design, 3E McGraw-Hill Small-Signal Operation of Diode (cont.) Subtracting I D from both sides of the equation, For i d to be a linear function of signal voltage v d, This represents the requirement for small-signal operation of the diode.

5. Microelectronic Circuit Design, 3E McGraw-Hill Current-Controlled Attenuator Magnitude of ac voltage v o developed across diode can be controlled by value of dc bias current applied to diode. From dc equivalent circuit I D = I, From ac equivalent circuit, For R I = 1 k , I S = 10 -15 A, If I = 0, v o = v i, magnitude of v i is limited to only 5 mV. If I = 100  A, input signal is attenuated by a factor of 5, and v i can have a magnitude of 25 mV.

6. Microelectronic Circuit Design, 3E McGraw-Hill Small-Signal Model of BJT Using 2-port y-parameter network, The port variables can represent either time-varying part of total voltages and currents or small changes in them away from Q-point values.  o is the small-signal common- emitter current gain of the BJT.

7. Microelectronic Circuit Design, 3E McGraw-Hill Hybrid-Pi Model of BJT The hybrid-pi small-signal model is the intrinsic representation of the BJT. Small-signal parameters are controlled by the Q-point and are independent of geometry of the BJT Transconductance: Input resistance: Output resistance:

8. Microelectronic Circuit Design, 3E McGraw-Hill Small-Signal Current Gain and Amplification Factor of BJT  o >  F for i C I M, however,  F and  o are assumed to be equal. Amplification factor is given by: For V CE << V A,  F represents maximum voltage gain individual BJT can provide and doesn’t change with operating point.

9. Microelectronic Circuit Design, 3E McGraw-Hill Equivalent Forms of Small-Signal Model for BJT Voltage -controlled current source g m v be can be transformed into current-controlled current source, Basic relationship i c =  i b is useful in both dc and ac analysis when BJT is in forward-active region.