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Class A BJT Amplifiers
62 questions By Tony R. Kuphaldt
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Question 19 of 62
Predict how all transistor currents (IB, IC, and IE) 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 Cin fails open:
- Solder bridge (short) past resistor R1:
- Resistor R1 fails open:
- Resistor RC fails open:
- Resistor RE fails open:
- Capacitor Cbypass fails shorted:
For each of these conditions, explain why the resulting effects will occur.
Reveal answer- Capacitor Cin 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 RC fails open: Transistor base current will decrease, zero collector current, greatly decreased emitter current, no output signal.
- Resistor RE fails open: All transistor currents fall to zero (transistor in complete cutoff mode), no output signal.
- Capacitor Cbypass fails shorted: Transistor saturates (large increase in all currents), 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.
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Question 20 of 62
Complete the table of voltages and currents for several given values of input voltage in this common-base amplifier circuit. Assume that the transistor is a standard silicon NPN unit, with a nominal base-base junction forward voltage of 0.7 volts:
VE VB IB IC VRC VC 0.0 V -0.5 V -0.8 V -1.0 V -1.1 V -1.2 V -1.3 V
Calculate the voltage gain of this circuit from the numerical values in the table:$$A_V=\frac{\triangle V_{out}}{\triangle V_in}}=$$
Reveal answerVE VB IB IC VRC VC 0.0 V 0.0 V 0.0 μA 0.0 mA 0.0 V 15 V -0.5 V 0.0 V 0.0 μA 0.0 mA 0.0 V 15 V -0.8 V -0.1 V 45.5 μA 2.27 mA 2.27 V 12.7 V -1.0 V -0.3 V 136.4 μA 6.82 mA 6.82 V 8.18 V -1.1 V -0.4 V 181.8 μA 9.09 mA 9.09 V 5.91 V -1.2 V -0.5 V 227.3 μA 11.36 mA 11.36 V 3.64 V -1.3 V -0.6 V 272.7 μA 13.64 mA 13.64 V 1.36 V $$A_V=\frac{\triangle V_{out}}{\triangle V_in}}=22.7$$
Follow-up question: based on the values for output and input voltage shown in the table, would you say that common-base amplifier circuits are inverting or noninverting?
Notes:The purpose of this question, besides providing practice for common-base circuit DC analysis, is to show the noninverting and voltage-amplification properties of the common-base amplifier, as well as to showcase its low current gain.
The negative values shown for emitter voltage (Vin) are correct and intentional. It is necessary to view the input voltage as a negative quantity to confidently determine the phase relationship between input and output.
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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Question 21 of 62
Describe the functions of resistors R1 and R2, and capacitor C1, in this amplifier circuit. What purpose do they serve?
Would it be possible for this amplifier circuit to operate in Class A mode without this voltage divider/capacitor network? Explain your answer.
Reveal answerResistors R1 and R2 produce a DC biasing voltage, and capacitor C1 “couples” the AC input signal to the biasing network, preventing any DC bias from the signal source to influence the amplifier’s Q point.
While other biasing networks are possible for a Class A amplifier circuit, some type of DC bias voltage is necessary to make a bipolar junction transistor operate in Class A mode.
Notes:Discuss with your students the meaning of “Class A” amplification, and why DC biasing is necessary in order to achieve this mode of operation in a BJT circuit.
