Download Bipolar transistor biasing circuits and more Exams Reasoning in PDF only on Docsity!
Bipolar transistor biasing circuits
This worksheet and all related files are licensed under the Creative Commons Attribution License, version 1.0. To view a copy of this license, visit http://creativecommons.org/licenses/by/1.0/, or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. The terms and conditions of this license allow for free copying, distribution, and/or modification of all licensed works by the general public.
Resources and methods for learning about these subjects (list a few here, in preparation for your research):
Questions
Question 1
Describe what the output voltage of this transistor circuit will do (measured with reference to ground), if the potentiometer wiper begins at the full-down position (common with ground), and is slowly moved in the upward direction (closer to +V):
+V
Vout
10 V
file 02220
Question 2
Complete the table of output voltages for several given values of input voltage in this common-collector amplifier circuit. Assume that the transistor is a standard silicon NPN unit, with a nominal base-emitter junction forward voltage of 0.7 volts:
Vin Vout
+15 V
1.5 kΩ
Vin Vout 0.0 V 0.5 V 1.0 V 1.5 V 5.0 V 7.8 V
Based on the values you calculate, explain why the common-collector circuit configuration is often referred to as an emitter follower. file 02224
Question 6
Class-A operation may be obtained from this simple transistor circuit if the input voltage (Vin) is ”biased” with a series-connected DC voltage source:
+V
Vin
Vbias
First, define what ”Class A” amplifier operation is. Then, explain why biasing is required for this transistor to achieve it. file 02223
Question 7
Describe what the output voltage of this transistor circuit will do (measured with reference to ground), if the potentiometer wiper begins at the full-down position (common with ground), and is slowly moved in the upward direction (closer to +V):
+V
Vout
10 V
file 00822
Question 8
If we were to apply a sinusoidal AC signal to the input of this transistor amplifier circuit, the output would definitely not be sinusoidal:
+V
It should be apparent that only portions of the input are being amplified in this circuit. The rest of the waveform seems to be ”missing” in the output, being replaced by a flat line. Explain why this transistor circuit is not able to amplify the entire waveform. file 00746
Question 9
Class-A operation may be obtained from this simple transistor circuit if the input voltage (Vin) is ”biased” with a series-connected DC voltage source:
+V
Vin
Vbias
Vout
First, define what ”Class A” amplifier operation is. Then, explain why biasing is required for this transistor to achieve it. file 00747
How can this amplifier circuit be producing such a distorted output waveform with such a clean input waveform? Explain your answer. file 00748
Question 11
Suppose you were building a Class-A transistor amplifier for audio frequency use, but did not have an oscilloscope available to check the output waveform for the presence of ”clipping” caused by improper biasing. You do, however, have a pair of audio headphones you may use to listen to the signals. Explain how you would use a pair of headphones to check for the presence of severe distortion in a waveform. file 00751
Question 12
Explain how it is possible for a fault in the biasing circuitry of a transistor amplifier to completely kill the (AC) output of that amplifier. How and why can a shift in DC bias voltage have an effect on the AC signal being amplified? file 03741
Question 13
Calculate the approximate quiescent (DC) base current for this transistor circuit, assuming an AC input voltage of 0 volts, and a silicon transistor:
Vin
Vbias
Vout
47 kΩ
-20 V
2.5 V
10 kΩ
file 00823
Question 14
Calculate the potentiometer wiper voltage (Vbias) required to maintain the transistor right at the threshold between cutoff and active mode. Then, calculate the input voltage required to drive the transistor right to the threshold between active mode and saturation. Assume ideal silicon transistor behavior, with a constant β of 100:
Vout
25 V
-V
1 kΩ
Vbias
33 kΩ
8.1 kΩ
file 00824
Question 16
Explain how the following bias networks function:
+V
Vin
Vout
-V
Emitter bias
+V
Vin
Vout
Base bias
Voltage divider bias^ +V +V
Vin
Vout
Voltage divider bias
(when Vin is not
ground-referenced)
Vin
Vout
Each one has the same basic purpose, but works in a different way to accomplish it. Describe the purpose of any biasing network in an AC signal amplifier, and comment on the different means of accomplishing this purpose employed by each of the three circuits. file 00749
Question 17
A very common method of providing bias voltage for transistor amplifier circuits is with a voltage divider:
+V
Vout
Voltage divider bias
However, if we were to directly connect a source of AC signal voltage to the junction between the two voltage divider resistors, the circuit would most likely function as if there were no voltage divider network in place at all:
+V
Vout
+V
Vout
Equivalent to
Instead, circuit designers usually place a coupling capacitor between the signal source and the voltage divider junction, like this:
Question 19
Describe how proper biasing is accomplished in this headphone amplifier circuit (suitable for amplifying the audio output of a small radio):
Headphones
Signal input
1000 μF
22 μF
6 V
100 kΩ
10 kΩ
Also, describe the functions of the 10 kΩ potentiometer and the 22 μF capacitor. file 00750
Question 20
The following circuit is a three-channel audio mixer circuit, used to blend and amplify three different audio signals (coming from microphones or other signal sources):
1 kΩ
10 kΩ
81 kΩ
Vin
Vin
Vin
50 kΩ
50 kΩ
50 kΩ
47 μF
100 μF
220 μF
Vout
A 33 kΩ
4.7 kΩ
B
VCC = +12 VDC
Suppose we measured a 9 kHz sinusoidal voltage of 0.5 volts (peak) at point ”A” in the diagram, using an oscilloscope. Determine the voltage at point ”B” in the circuit, after this AC signal voltage ”passes through” the voltage divider biasing network. The voltage at point ”B” will be a mix of AC and DC, so be sure to express both quantities! Ignore any ”loading” effects of the transistor’s base current on the voltage divider. file 00825
Question 21
Don’t just sit there! Build something!!
Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way. You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the ”answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:
- Carefully measure and record all component values prior to circuit construction, choosing resistor values high enough to make damage to any active components unlikely.
- Draw the schematic diagram for the circuit to be analyzed.
- Carefully build this circuit on a breadboard or other convenient medium.
- Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
- Mathematically analyze the circuit, solving for all voltage and current values.
- Carefully measure all voltages and currents, to verify the accuracy of your analysis.
- If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction against the diagram, then carefully re-calculate the values and re-measure. When students are first learning about semiconductor devices, and are most likely to damage them by making improper connections in their circuits, I recommend they experiment with large, high-wattage components (1N4001 rectifying diodes, TO-220 or TO-3 case power transistors, etc.), and using dry-cell battery power sources rather than a benchtop power supply. This decreases the likelihood of component damage. As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter ”loading” (on the high end) and to avoid transistor burnout (on the low end). I recommend resistors between 1 kΩ and 100 kΩ. One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once. file 00505
Answer 9
”Class A” amplifier operation is when the transistor remains in its ”active” mode (conducting current) throughout the entire waveform. Biasing may be thought of as a kind of ”trick” used to get the transistor (a DC device) to ”think” it is amplifying DC when the input signal is really AC.
Answer 10
The DC bias voltage (Vbias) is excessive.
Answer 11
Set the signal generator to ”sine-wave,” and the aural difference between a pure sine wave and a distorted (”clipped”) sine wave will be very apparent.
Answer 12
If the DC bias voltage shifts far enough away from the normal (quiescent) levels, the transistor may be forced into saturation or cutoff so it cannot reproduce the AC signal.
Answer 13
IB = 38.3 μA
Answer 14
At the threshold between cutoff and active mode, Vbias = -0.7 volts
At the threshold between active mode and saturation, Vbias = -1.72 volts (assuming 0 volts VCE at saturation)
Follow-up question: if we were using the potentiometer to establish a bias voltage for an AC signal, what amount of DC bias voltage would place the transistor directly between these two extremes of operation (cutoff versus saturation), so as to allow the AC input signal to ”swing” equal amounts positive and negative at the distortion limit? In other words, what voltage setting is exactly between -0.7 volts and -1.72 volts?
Answer 15
The purpose of any biasing network in an AC signal amplifier is to provide just enough quiescent current through the base to keep the transistor between the extremes of cutoff and saturation throughout the input signal’s waveform cycle.
Answer 16
The purpose of any biasing network in an AC signal amplifier is to provide just enough quiescent current through the base to keep the transistor between the extremes of cutoff and saturation throughout the input signal’s waveform cycle.
Answer 17
A very good way to understand the AC source’s effect on the voltage divider with and without the capacitor is to use Superposition Theorem to determine what each source (AC signal, and DC power supply) will do separately. If this concept is still not clear, consider this circuit:
Vin Vbias
Determine the voltage
at this point
As far as capacitor size is concerned, it should be large enough that its reactance is negligible. I’ll let you determine what factors define negligibility in this context!
Follow-up question: which voltage source (AC or DC?) ”wins” at the point specified in the above circuit? Explain why this is so, and then show how a suitably located capacitor would allow both voltage signals to co-exist at that point.
Answer 18
Vout
Answer 19
Biasing is accomplished through the 100 kΩ resistor. The 10 kΩ potentiometer is the volume control, and the 22 μF capacitor serves to ”couple” the input signal to the transistor’s base, while blocking any DC bias voltage from being ”fed back” to the audio signal source.
Challenge question: there is a name used to describe the dual-transistor configuration used in this circuit, where a pair of PNP or NPN transistors is cascaded, with the emitter of one going to the base of the other. What is this name, and what advantage does this configuration provide over a single transistor?
Answer 20
VB = 1.318 VDC + 0.5 VAC (peak)
Notes
Notes 1
Although this circuit is very simple, it is also very important to master. Be sure to discuss its operation thoroughly with your students, so they understand.
Notes 2
At first, the ”emitter follower” transistor circuit may seem pointless, since the output voltage practically equals the input voltage (especially for input voltages greatly exceeding 0.7 volts DC). ”What possible good is a circuit like this?” some of your students may ask. The answer to this question, of course, has to do with currents in the circuit, and not necessarily voltages.
Notes 3
This might not be the result many students expect! It is important, though, for them to understand the importance of polarity in transistor circuits. This example should make that abundantly clear.
Notes 4
Sometimes it is helpful for students to re-draw the circuit using a transistor model showing the base- emitter junction as a diode. If you think this model would help some of your students understand the concept here, have another student draw the transistor model on the whiteboard, and use that drawing as a discussion aid. Like any PN junction, the base-emitter junction of a BJT only ”wants” to conduct current in one direction.
Notes 5
Of course, the natural question following this one is, ”What other classes of operation are there?” This would be an excellent time to preview Class-B (push-pull) and Class-C operations if time permits.
Notes 6
A ”trick” it may be, but a very useful and very common ”trick” it is! Discuss this concept with your students at length, being sure they have ample time and opportunity to ask questions of their own. One question that may arise is, ”how much DC bias voltage is necessary?” If no one asks this question, ask it yourself! Discuss with your students what would constitute the minimum amount of bias voltage necessary to ensure the transistor never goes into ”cutoff” anywhere in the waveform’s cycle, and also the maximum bias voltage to prevent the transistor from ”saturating”.
Notes 7
Although this circuit is very simple, it is also very important to master. Be sure to discuss its operation thoroughly with your students, so they understand.
Notes 8
Sometimes it is helpful for students to re-draw the circuit using a transistor model showing the base- emitter junction as a diode. If you think this model would help some of your students understand the concept here, have another student draw the transistor model on the whiteboard, and use that drawing as a discussion aid. Like any PN junction, the base-emitter junction of a BJT only ”wants” to conduct current in one direction.
Notes 9
A ”trick” it may be, but a very useful and very common ”trick” it is! Discuss this concept with your students at length, being sure they have ample time and opportunity to ask questions of their own. One question that may arise is, ”how much DC bias voltage is necessary?” If no one asks this question, ask it yourself! Discuss with your students what would constitute the minimum amount of bias voltage necessary to ensure the transistor never goes into ”cutoff” anywhere in the waveform’s cycle, and also the maximum bias voltage to prevent the transistor from ”saturating”.
Notes 10
Ask your students how they can tell the difference between excessive biasing and insufficient biasing, by inspection of the output waveform. There is a difference to be seen, but it requires a good understanding of how the circuit works! Students may be tempted to simply memorize waveforms (”when I see this kind of waveform, I know the problem is excessive biasing.. .”), so prepare to challenge their understanding with questions such as:
- What polarity of input signal drives the transistor toward cutoff?
- What polarity of input signal drives the transistor toward saturation?
- Where on the output waveform is the transistor in cutoff (if at all)?
- Where on the output waveform is the transistor in saturation (if at all)?
- Where on the output waveform is the transistor in its active mode? Another point worth mentioning: some students may be confused by the phasing of the input and output waveforms, comparing the two different oscilloscope displays. For a common-emitter (inverting) amplifier such as this, they expect to see the output voltage peak positive whenever the input voltage peaks negative, and visa-versa, but here the two oscilloscope displays show positive peaks occurring right next to the left-hand side of the screen. Why is this? Because the oscilloscope does not represent phase unless it is in dual-trace mode! When you disconnect the input probe and move it to another point in the circuit, any time reference is lost, the oscilloscope’s triggering function placing the first waveform peak right where you tell it to, usually near the left-hand side of the display.
Notes 11
The answer I want for this question is not just a parroting of the answer I’ve given. Anyone can say ”a distorted wave will sound different.” I want to know how it sounds different, and this answer can only come by direct experimentation!
Notes 12
This question asks students to explore the possibility of complete AC signal failure due to a simple shift in DC bias, based on their understanding of how transistor amplifiers function. It may seem paradoxical that such a ”small” fault could have such a large effect on an amplifier circuit, but it should make sense once students grasp how important bias is to class-A amplifier operation.
Notes 13
This circuit was purposely drawn in a convoluted fashion to force students to identify its configuration apart from the standard layout. Many people lack the spatial reasoning skills to do this easily, and require a lot of practice before they become proficient. Ask your more proficient students if they have any ”tips” for helping those who struggle with problems like these. Are there any simple methods which we may use to re-draw this circuit in an easier-to-understand form?
