Discrete Semiconductor Devices and Circuits

Class A BJT Amplifiers

62 questions By Tony R. Kuphaldt

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Question 7 of 62

Calculate the approximate voltage gain (A_V) for the following common-collector amplifier circuit, expressing it as a ratio and as a decibel value. Also calculate the quiescent DC voltage measured across the load resistor (V_load(DC)). Assume a silicon transistor:

A_V (as a ratio) ≈

A_V (in decibels) ≈

V_load(DC) ≈

Reveal answer

A_V (as a ratio) ≈ 1

A_V (in decibels) ≈ 0 dB

V_load(DC) ≈ 6.8 volts

Notes:
Nothing much to comment on here - just some practice on common-collector amplifier calculations. Note that the approximations given here are based on the following assumptions:

0.7 volts drop (exactly) across base-emitter junction.

Infinite DC current gain (β) for transistor (I_B = 0 μA ; I_C = I_E).

Negligible loading of bias voltage divider by the emitter resistance.

Negligible dynamic emitter resistance (r′_e = 0 Ω )

This question lends itself well to group discussions on component failure scenarios. After discussing how to calculate the requested values, you might want to ask students to consider how these values would change given some specific component failures (open resistors, primarily, since this is perhaps the most common way that a resistor could fail).
Question 8 of 62

Predict how all transistor currents (I_B, I_C, and I_E) and the output voltage signal will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Capacitor C_in fails open:

Solder bridge (short) past resistor R₁:

Resistor R₁ fails open:

Resistor R_E fails open:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Capacitor C_in fails open: All transistor currents assume quiescent (DC) values, no output signal.

Solder bridge (short) past resistor R₁: Transistor saturates (large increase in all currents), no output signal.

Resistor R₁ fails open: All transistor currents fall to zero (transistor in complete cutoff mode), no output signal.

Resistor R_E fails open: All transistor currents fall to zero (transistor in complete cutoff mode), no output signal.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.

Question 9 of 62

Complete the table of voltages and currents for several given values of input voltage in this common-emitter amplifier circuit. Assume that the transistor is a standard silicon NPN unit, with a nominal base-emitter junction forward voltage of 0.7 volts. For the last row of the table, give qualitative answers (increase, decrease, or same) representing what each of the quantities will do given an increasing base voltage (V_B):

V_B	V_E	I_C	V_{R_C}	V_CE	V_C (V_out)
0.0 V
0.5 V
1.0 V
1.5 V
2.0 V
2.5 V
3.0 V
increase

Calculate the voltage gain of this circuit from the numerical values in the table:

$$A_V=\frac{\triangle V_{out}}{\triangle V_{in}}=$$

Reveal answer

V_B	V_E	I_C	V_{R_C}	V_CE	V_C (V_out)
0.0 V	0.0 V	0.0 mA	0.0 V	15 V	15 V
0.5 V	0.0 V	0.0 mA	0.0 V	15 V	15 V
1.0 V	0.3 V	0.298 mA	1.40 V	13.3 V	13.6 V
1.5 V	0.8 V	0.793 mA	3.73 V	10.47 V	11.27 V
2.0 V	1.3 V	1.29 mA	6.06 V	7.64 V	8.94 V
2.5 V	1.8 V	1.79 mA	8.39 V	4.81 V	6.61 V
3.0 V	2.3 V	2.28 mA	10.7 V	1.98 V	4.28 V
increase	increase	increase	increase	decrease	decrease

$$A_V=\frac{\triangle V_{out}}{\triangle V_{in}}=4.66$$

Sometimes the voltage gain of a common-emitter amplifier circuit is expressed as a negative quantity (-4.66 in this case), to indicate the inverse output/input relationship (180^o phase shift).

Follow-up question: what similarity do you notice between the voltage gain value of 4.66 and the two resistor values?

Challenge question: a common assumption used in this type of BJT amplifier circuit is I_C ≈ I_E. Develop a voltage gain formula based on this assumption, in terms of resistor values R_C and R_E.

Notes:

The purpose of this question, besides providing practice for common-emitter circuit DC analysis, is to show the signal-inverting and voltage-amplification properties of the common-emitter amplifier. Some students experience difficulty understanding why V_C (the output voltage) decreases with increasing base voltage (V_B). Working through the numbers in this table gives concrete proof why it is so.

This approach to determining transistor amplifier circuit voltage gain is one that does not require prior knowledge of amplifier configurations. In order to obtain the necessary data to calculate voltage gain, all one needs to know are the “first principles” of Ohm’s Law, Kirchhoff’s Laws, and basic operating principles of a bipolar junction transistor. This question is really just a thought experiment: exploring an unknown form of circuit by applying known rules of circuit components. If students doubt the efficacy of “thought experiments,” one need only to reflect on the success of Albert Einstein, whose thought experiments as a patent clerk (without the aid of experimental equipment) allowed him to formulate the basis of his Theories of Relativity.

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