Chapter 29

	Transistor Amplifiers

Use of Capacitors in Amplifier Circuits

	Capacitor review
		Store electrical charge
		Impedance:
		∞ impedance at dc
		Impedance decreases at higher frequencies

Use of Capacitors in Amplifier Circuits

	Capacitors
		Block dc between stages
		Can be designed to readily pass ac

Use of Capacitors in Amplifier Circuits

	Coupling capacitors
		At “high” frequencies
		For R = Rin + RS, select capacitor so XC ≤ 0.1 R
		Referred to as “stiff coupling”

Use of Capacitors in Amplifier Circuits

	Bypass capacitors
		Emitter resistor, Re used for biasing
		Ce is a short circuit at high frequencies
		Re has no effect on amplification when Ce is present
		Select XC ≤ 0.1R

Use of Capacitors in Amplifier Circuits

Use of Capacitors in Amplifier Circuits

	Capacitors
		Couple desired ac signals between stages
		Bypass unwanted ac signals to ground

Use of Capacitors in Amplifier Circuits

	Circuit analysis
		If XC ≤ 0.1R
		Replace C with O.C. to determine dc I and V
		Replace C with S.C. to determine ac i and v

BJT Small-Signal Models

	T-Equivalent Model
		ie = ib + ic
		ie = (β + 1)ib
		Simple
		Good enough for most applications

BJT Small-Signal Models

BJT Small-Signal Models

	Models
		T-equivalent model simpler
		h-parameter model more accurate
		hfe (h-model) = βac (T-model) [βac ≈ βdc]
		h-parameters dependent on Q-point
		BJT is a current amplifier (current source in both models)

BJT Small-Signal Models

	h-parameter model
		More complex
		Better for ac operation
		Common Emitter model
			hie = input impedance (Ω)
			hre = reverse voltage transfer ratio (unitless)
			hfe = forward current transfer ratio (unitless)
			hoe = output admittance (S)

Calculating A_v, z_in, z_out, and A_i of a Transistor Amplifier

	Voltage Gain, Av
		Output voltage divided by input voltage
	Input Impedance, zin
		Input voltage divided by input current

Calculating A_v, z_in, z_out, and A_i of a Transistor Amplifier

	Output Impedance, zout
	Current Gain, Ai
	Power Gain, Ap

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

Common-Emitter Amplifier

	ac equivalent of fixed-bias CE amplifier using h-parameter model

Common-Emitter Amplifier

	Equations for h-parameter model for fixed-bias CE amplifier
		Circuit voltage gain a function of
			Model forward current transfer ratio, hfe
			Model input impedance, hie
			Circuit collector resistance, RC
			Circuit load resistance, RL

Common-Emitter Amplifier

	Circuit current gain a function of
		Same parameters, plus
		Fixed bias resistance, RB

Common-Emitter Amplifier

	Equations for h-parameter model for fixed-bias CE amplifier
		Circuit input impedance a function of
			Model forward current transfer ratio, hfe
			Model input impedance, hie

Common-Emitter Amplifier

	Circuit output impedance a function of
		Collector resistance (model output admittance), hoe very low

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

ac Load Line

	CE amplifier circuit

ac Load Line

	dc and ac load lines

ac Load Line

	Equations of ac load line
	Consider
		CE amplifier circuit
		dc load line

Common-Collector Amplifier

	Important characteristics
		High input impedance
		Low output impedance
		vout in-phase with vin
		vout ≈ vin

Common-Collector Amplifier

	Important characteristics
		Large current gain
		Input voltage measured at base
		Output voltage measured at emitter

Common-Collector Amplifier

	Common-Collector circuit

Common-Collector Amplifier

	Circuit gains and impedances
		Av ≈ 1
		zin = RB||zin(Q)
		close to hfe
		very small

FET Small-Signal Model

	Voltage controlled amplifier
	Small-signal model same for JFETs & MOSFETs
	High input impedance
	is = id

FET Small-Signal Model

	gm is transconductance
	gm is slope of transfer curve

FET Small-Signal Model

	Equations
		Definition
		Maximum
		Measured

Common-Source Amplifier

	Analysis
		Similar to BJT using h-parameter model
		First determine bias
		Find dc operating point (Q-point)
		Determine gm

Common-Source Amplifier

	A common-source circuit

Common-Source Amplifier

	Equations
		No current input
		Voltage gain dependent on gm  and RD
		Input impedance is RG || ∞
		Output impedance approximately drain resistance

Common-Source Amplifier

	D-MOSFETs
		Analysis same as JFETs
		Except operation in enhancement region

Common-Source Amplifier

	E-MOSFETs
		Find IDSQ, VGSQ, and VDSQ at Q-point
		Solve for gm of amplifier
		Sketch ac equivalent circuit
		Determine Av, zin, and zout of amplifier

Common-Drain (Source Follower) Amplifier

	Av < 1
	vout in phase with vin
	Input impedance very high
	Output impedance low
	Main application: Buffer

Troubleshooting a Transistor Amplifier Circuit

	Incorrect placement of electrolytic capacitors
		Noisy output signal
		Capacitor as an antenna
		Generally 60 Hz added

Troubleshooting a Transistor Amplifier Circuit

	Correct placement
		Check proper polarity
		Replace faulty capacitors

Troubleshooting a Transistor Amplifier Circuit

	Faulty or incorrectly placed capacitor
		Measured Av different from theoretical Av
		Faulty capacitor behaves like an open circuit
		Faulty capacitor can develop internal short

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

Troubleshooting a Transistor Amplifier Circuit

	Distorted output signal usually the result of too large an input signal