1	Chapter 29 Transistor Amplifiers
2	Use of Capacitors in Amplifier Circuits Capacitor review Store electrical charge Impedance: ∞ impedance at dc Impedance decreases at higher frequencies
3	Use of Capacitors in Amplifier Circuits Capacitors Block dc between stages Can be designed to readily pass ac
4	Use of Capacitors in Amplifier Circuits Coupling capacitors At “high” frequencies For R = R_in + R_S, select capacitor so X_C ≤ 0.1 R Referred to as “stiff coupling”
5	Use of Capacitors in Amplifier Circuits Bypass capacitors Emitter resistor, R_e used for biasing C_e is a short circuit at high frequencies R_e has no effect on amplification when C_e is present Select X_C ≤ 0.1R
6	Use of Capacitors in Amplifier Circuits
7	Use of Capacitors in Amplifier Circuits Capacitors Couple desired ac signals between stages Bypass unwanted ac signals to ground
8	Use of Capacitors in Amplifier Circuits Circuit analysis If X_C ≤ 0.1R Replace C with O.C. to determine dc I and V Replace C with S.C. to determine ac i and v
9	BJT Small-Signal Models T-Equivalent Model i_e = i_b + i_c i_e = (β + 1)i_b Simple Good enough for most applications
10	BJT Small-Signal Models
11	BJT Small-Signal Models Models T-equivalent model simpler h-parameter model more accurate h_fe (h-model) = β_ac (T-model) [β_ac ≈ β_dc] h-parameters dependent on Q-point BJT is a current amplifier (current source in both models)
12	BJT Small-Signal Models h-parameter model More complex Better for ac operation Common Emitter model h_ie = input impedance (Ω) h_re = reverse voltage transfer ratio (unitless) h_fe = forward current transfer ratio (unitless) h_oe = output admittance (S)
13	Calculating A_v, z_in, z_out, and A_i of a Transistor Amplifier Voltage Gain, A_v Output voltage divided by input voltage Input Impedance, z_in Input voltage divided by input current
14	Calculating A_v, z_in, z_out, and A_i of a Transistor Amplifier Output Impedance, z_out Current Gain, A_i Power Gain, A_p
15	Common-Emitter Amplifier General BJT circuit analysis Find operating point Determine ac parameters (T- or h- models) Remove dc V sources & replace with S.C.’s Replace coupling & bypass C’s with S.C.’s Replace BJT with circuit model Solve resulting circuit
16	Common-Emitter Amplifier ac equivalent of fixed-bias CE amplifier using h-parameter model
17	Common-Emitter Amplifier Equations for h-parameter model for fixed-bias CE amplifier Circuit voltage gain a function of Model forward current transfer ratio, h_fe Model input impedance, h_ie Circuit collector resistance, R_C Circuit load resistance, R_L
18	Common-Emitter Amplifier Circuit current gain a function of Same parameters, plus Fixed bias resistance, R_B
19	Common-Emitter Amplifier Equations for h-parameter model for fixed-bias CE amplifier Circuit input impedance a function of Model forward current transfer ratio, h_fe Model input impedance, h_ie
20	Common-Emitter Amplifier Circuit output impedance a function of Collector resistance (model output admittance), h_oe very low
21	ac Load Line Q-point is on dc load line ac load line determines maximum undistorted output Can calculate maximum power Q-point also on ac load line ac load line has different slope
22	ac Load Line CE amplifier circuit
23	ac Load Line dc and ac load lines
24	ac Load Line Equations of ac load line Consider CE amplifier circuit dc load line
25	Common-Collector Amplifier Important characteristics High input impedance Low output impedance v_out in-phase with v_in v_out ≈ v_in
26	Common-Collector Amplifier Important characteristics Large current gain Input voltage measured at base Output voltage measured at emitter
27	Common-Collector Amplifier Common-Collector circuit
28	Common-Collector Amplifier Circuit gains and impedances A_v ≈ 1 z_in = R_B\|\|z_in(Q) close to h_fe very small
29	FET Small-Signal Model Voltage controlled amplifier Small-signal model same for JFETs & MOSFETs High input impedance i_s = i_d
30	FET Small-Signal Model g_m is transconductance g_m is slope of transfer curve
31	FET Small-Signal Model Equations Definition Maximum Measured
32	Common-Source Amplifier Analysis Similar to BJT using h-parameter model First determine bias Find dc operating point (Q-point) Determine g_m
33	Common-Source Amplifier A common-source circuit
34	Common-Source Amplifier Equations No current input Voltage gain dependent on g_m and R_D Input impedance is R_G \|\| ∞ Output impedance approximately drain resistance
35	Common-Source Amplifier D-MOSFETs Analysis same as JFETs Except operation in enhancement region
36	Common-Source Amplifier E-MOSFETs Find I_DSQ, V_GSQ, and V_DSQ at Q-point Solve for g_m of amplifier Sketch ac equivalent circuit Determine A_v, z_in, and z_out of amplifier
37	Common-Drain (Source Follower) Amplifier A_v < 1 v_out in phase with v_in Input impedance very high Output impedance low Main application: Buffer
38	Troubleshooting a Transistor Amplifier Circuit Incorrect placement of electrolytic capacitors Noisy output signal Capacitor as an antenna Generally 60 Hz added
39	Troubleshooting a Transistor Amplifier Circuit Correct placement Check proper polarity Replace faulty capacitors
40	Troubleshooting a Transistor Amplifier Circuit Faulty or incorrectly placed capacitor Measured A_v different from theoretical A_v Faulty capacitor behaves like an open circuit Faulty capacitor can develop internal short
41	Troubleshooting a Transistor Amplifier Circuit Troubleshooting steps Remove ac signal sources from circuit Calculate theoretical Q-point Measure to determine actual Q-point Verify capacitors are correctly placed Ensure connections, especially ground wires, as short as possible
42	Troubleshooting a Transistor Amplifier Circuit Distorted output signal usually the result of too large an input signal