Small Signal Analysis in Transistor Amplifiers Exam Questions Answered Correctly Latest 2025
Transistor small-signal amplifiers - Answers Can be considered to operate linear for most applications,
permitting the use of the superposition theorem to isolate the dc analysis from the ac analysis.
Model (ac-modelling) - Answers A combination of circuit elements, properly chosen, that best
approximates the actual behaviour of a semiconductor device under specific operating conditions.
Two-port model - Answers Used to define the equivalent circuit for BJT's or FET's, consisting of two pairs
of terminals: an input side for the signal and an output side for the load.
Input impedance (Zi) - Answers Defined as Zi = Vi / Ii, and can vary from a few ohms to megohms
depending on the transistor usage.
Output impedance (Zo) - Answers Determined at the output terminals looking back into the system with
the applied voltage set to zero, defined as Zo = Vo / Io.
No-load voltage gain (Av(NL)) - Answers Defined by Av(NL) = Vo / Vi when no load is connected to the
output terminals.
Voltage Gain (Av) - Answers Defined as Av = Vo / Vi.
Current Gain (Ai) - Answers Defined as Ai = Io / Ii, typically ranging from just less than 1 to exceeding 100
for BJT amplifiers.
Voltage ratio in decibel - Answers The voltage ratio in decibel between points A and B with no load
connected is -6.02 dB.
RMS voltage between A and B - Answers 80mV RMS when R1 is 200 Ω, changes to 120 mV RMS when R1
is an open circuit.
Peak voltage of Vs1 - Answers Determined to be 169.7 mV.
RMS voltage with R1 equal to 400 Ω - Answers The voltage between A and B with R1 as 400 Ω is to be
determined, with Vs1 and rs unchanged.
Output voltage (Vo) - Answers In the context of Ai, defined as Io = -Vo / RL.
Input voltage (Vi) - Answers In the context of Ai, defined as Ii = Vi / Zi.
Resistance (Rsource) - Answers The effect of the source resistance in the voltage gain equation is
determined by Vi = Zi.Vs / (Zi + Rs).
Resistance (rs) - Answers Given as 100Ω.
Load resistor (RL) - Answers Given as 500 Ω.
Internal resistance (rs) - Answers Given as 1000 Ω.
, Voltage between C and D - Answers 800mV RMS while the voltage between A and B is 40mV RMS.
Voltage between C and D when open circuit - Answers Rises to 1000 mV RMS when the load resistor RL
is removed.
Current gain formula - Answers Ai = -Av. Zi / RL.
Zo - Answers 125 Ω
Voltage Gain of Amplifier 1 with RL disconnected - Answers 25
Voltage Gain of Amplifier 1 with RL connected - Answers 20
Zi when voltage rises to 50 mV RMS - Answers 4000 Ω
RMS current value at point C with RL connected - Answers 1.6 mA (RMS)
Voltage gain of Amplifier 1 in decibel with RL connected - Answers 26 dB
re transistor model - Answers Common base configuration
re calculation - Answers re = VT / IE
VT - Answers k.T where k = Boltzmann's constant (1.379 X 10-23 J/K) and T = temperature in Kelvin
Input impedance Zi in common-base configuration - Answers Typical values range from a few ohms to a
maximum of about 50 ohm.
Output impedance with ie = 0A - Answers Zo = 4 Ω
Voltage gain Av - Answers Av = Vo / Vi
Vo in terms of Io and RL - Answers Vo = Io.RL = ie.RL
Vi in terms of ie and re - Answers Vi = ie.re
Current gain Ai - Answers Ai = io / ii = α.ie / ie
Input impedance in common emitter configuration - Answers Zi = Vi / Ii
Zo in common emitter configuration - Answers Zo = rO • VA / ICQ where VA = Early voltage
Hybrid parameter model equations - Answers Vi = h11.Ii + h12.Vo and Io = h21.Ii + h22.Vo
h11 - Answers Input resistance ÷ hi
h12 - Answers Reverse transfer ratio ÷ hr
h21 - Answers Forward transfer current ratio ÷ hf
Transistor small-signal amplifiers - Answers Can be considered to operate linear for most applications,
permitting the use of the superposition theorem to isolate the dc analysis from the ac analysis.
Model (ac-modelling) - Answers A combination of circuit elements, properly chosen, that best
approximates the actual behaviour of a semiconductor device under specific operating conditions.
Two-port model - Answers Used to define the equivalent circuit for BJT's or FET's, consisting of two pairs
of terminals: an input side for the signal and an output side for the load.
Input impedance (Zi) - Answers Defined as Zi = Vi / Ii, and can vary from a few ohms to megohms
depending on the transistor usage.
Output impedance (Zo) - Answers Determined at the output terminals looking back into the system with
the applied voltage set to zero, defined as Zo = Vo / Io.
No-load voltage gain (Av(NL)) - Answers Defined by Av(NL) = Vo / Vi when no load is connected to the
output terminals.
Voltage Gain (Av) - Answers Defined as Av = Vo / Vi.
Current Gain (Ai) - Answers Defined as Ai = Io / Ii, typically ranging from just less than 1 to exceeding 100
for BJT amplifiers.
Voltage ratio in decibel - Answers The voltage ratio in decibel between points A and B with no load
connected is -6.02 dB.
RMS voltage between A and B - Answers 80mV RMS when R1 is 200 Ω, changes to 120 mV RMS when R1
is an open circuit.
Peak voltage of Vs1 - Answers Determined to be 169.7 mV.
RMS voltage with R1 equal to 400 Ω - Answers The voltage between A and B with R1 as 400 Ω is to be
determined, with Vs1 and rs unchanged.
Output voltage (Vo) - Answers In the context of Ai, defined as Io = -Vo / RL.
Input voltage (Vi) - Answers In the context of Ai, defined as Ii = Vi / Zi.
Resistance (Rsource) - Answers The effect of the source resistance in the voltage gain equation is
determined by Vi = Zi.Vs / (Zi + Rs).
Resistance (rs) - Answers Given as 100Ω.
Load resistor (RL) - Answers Given as 500 Ω.
Internal resistance (rs) - Answers Given as 1000 Ω.
, Voltage between C and D - Answers 800mV RMS while the voltage between A and B is 40mV RMS.
Voltage between C and D when open circuit - Answers Rises to 1000 mV RMS when the load resistor RL
is removed.
Current gain formula - Answers Ai = -Av. Zi / RL.
Zo - Answers 125 Ω
Voltage Gain of Amplifier 1 with RL disconnected - Answers 25
Voltage Gain of Amplifier 1 with RL connected - Answers 20
Zi when voltage rises to 50 mV RMS - Answers 4000 Ω
RMS current value at point C with RL connected - Answers 1.6 mA (RMS)
Voltage gain of Amplifier 1 in decibel with RL connected - Answers 26 dB
re transistor model - Answers Common base configuration
re calculation - Answers re = VT / IE
VT - Answers k.T where k = Boltzmann's constant (1.379 X 10-23 J/K) and T = temperature in Kelvin
Input impedance Zi in common-base configuration - Answers Typical values range from a few ohms to a
maximum of about 50 ohm.
Output impedance with ie = 0A - Answers Zo = 4 Ω
Voltage gain Av - Answers Av = Vo / Vi
Vo in terms of Io and RL - Answers Vo = Io.RL = ie.RL
Vi in terms of ie and re - Answers Vi = ie.re
Current gain Ai - Answers Ai = io / ii = α.ie / ie
Input impedance in common emitter configuration - Answers Zi = Vi / Ii
Zo in common emitter configuration - Answers Zo = rO • VA / ICQ where VA = Early voltage
Hybrid parameter model equations - Answers Vi = h11.Ii + h12.Vo and Io = h21.Ii + h22.Vo
h11 - Answers Input resistance ÷ hi
h12 - Answers Reverse transfer ratio ÷ hr
h21 - Answers Forward transfer current ratio ÷ hf
