Design Article
Discrete audio amplifier basics - Part 1: Bipolar junction transistor circuits
John Linsley Hood
3/10/2010 2:00 PM EST
9.2 Control of Operating Bias
There are three basic ways of providing a DC quiescent voltage bias to a BJT, which is shown in Figure 9.4. In the first of these methods, shown in Figure 9.4(a), an arrangement that is fortunately seldom used, the method adopted is simply to connect an input resistor, R1, between the base of the transistor and some suitable voltage source. This voltage can then be adjusted so that the collector current of the transistor is of the right order to place the collector potential near its desired operating voltage.
Figure 9.4: Biasing circuits.
The snag with this scheme is that transistors vary quite a lot from one to another of nominally the same type, so this would require to be set anew for each individual device. Also, if the operating temperature changes, the current gain of the device (which is temperature sensitive) will be altered and, with it, the collector current of Q1 and its working potential.
The arrangement shown in Figure 9.4(b) is somewhat preferable in that a high current gain transistor, or one working at a higher temperature, will pass more current, and this will lower the collector voltage of Q1, which will, in turn, reduce the bias current flowing through R1. However, this also provides NFB and will limit the stage gain to a value somewhat less than R1/Zin.
The method almost invariably used in competently designed circuitry is that shown in Figure 9.4(c), or some equivalent layout. In this, a potential divider (R1,R2) having an output impedance low in relation to the base impedance of Q1 is used to provide a fixed DC base potential. Since the emitter will, by emitter"follower action, sit at a potential, depending on emitter current, which is about 0.6 V below that of the base, the value of R4 will then determine the emitter and collector currents, and the operating conditions so provided will hold good for almost any broadly similar device used in this position. Since the emitter resistor would cause a significant reduction in stage gain, as seen in the equivalent analysis of valve cathode bias systems, it is customary to bypass this resistor with a capacitor, C2, which is chosen to have an impedance low in relation to R4 and R3.
Next: Stage Gain
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abraxalito
3/10/2010 9:05 PM EST
I thought that Doug Self as editor would have caught the myth being propagated here that the BJT is a 'current operated device'. He makes plain in his own writings that its not, rather its voltage operated with an exponential law between VBE and IC.
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d-jeff
3/11/2010 3:32 AM EST
The bipolar IS voltage controlled with an exponential law (even though when a VBE is applied, some IB takes place). In this article on bipolar, this fact does not seem obvious, I agree with previous comment.
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bcarso
3/11/2010 12:17 PM EST
Hood was a delightful character, but wasn't always entirely clear on solid-state physics. I recall at one point in his description of JFET operation he refers to majority carriers "tunneling" through the pinched-off channel...
Although there are situations where a voltage-controlled analysis can be better suited to a given device, Barrie Gilbert makes the point that really both voltage and current are always involved (see his way-best-of-book articles in Toumazou's (et al., eds) collection Analogue IC Design: the current-mode approach). Another very powerful and seldom-referenced approach, analysis in terms of charge as the variable, is presented in Ed Cherry's Amplifying Devices and Low-pass Amplifier Design, which does an integrated presentation of tubes and transistors.
An aside: Barrie G. also mentioned once some experimental heterojunction ADI bipolars that had a beta of order 100k.
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Guru of Grounding
3/11/2010 7:32 PM EST
Arguing about whether a bipolar transistor is voltage or current "operated" is like arguing about how many angels can dance on the head of a pin! Notice that the context of his statement included the word "linearity". Many transistors have very linear hFE over many decades of current while Ic vs Vbe is anything but linear. Linearity is THE prime consideration in audio after all. Circuit designers simply want to characterize a transistor as a 3-terminal device and, for linear applications, the quantum physics involved inside are of little interest. I have disagreements with Mr. Hood, but they're about other issues. I think his explanation is quite appropriate in its context.
Bill Whitlock
president/chief engineer
Jensen Transformers
www.jensen-transformers.com
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bcarso
3/16/2010 4:06 PM EDT
Agreed Bill---although I would suggest that the expression should be "very constant beta" rather than "linear" beta, as a function of collector current. "Linear" beta for example would be where beta was given by an expression like m*Ic + b.
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c_abraham
6/12/2010 7:07 PM EDT
Then why does every semiconductor OEM call the bjt a current controlled device? Do you know more then them? How many semicond physics courses have you taken? Nothing personal, but if what you say is true, every OEM would concur, & every university semicond course would teach the same.
At the "black box" viewpoint, a bjt is classified as current controlled. For a deeper look involving internal charge distribution, depletion regions, etc., the charge control model is used, for the bjt & FET as well. Ultimately, at the atomic level, only quantum mechanics can describe what is happening. This is the universal model. Shockley stated this in the early 1950's.
Again, at the external viewpoint, current control is a good 1st order approximation. More in depth study produced the charge control model. Ultimately the device is best described by QM. Every OEM cannot be wrong. I took 1 undergrad course in semicond phy from the phy dept. Then I took 4 couses in semicond phyfrom the EE dept. in my MS & Ph.D. studies at 2 different universities. The bjt & FET are described as I just stated.
A bjt cannot be directly driven from a low impedance voltage source. Thermal runaway would take place. We always "current drive" a bjt. If the input signal source is constant voltage type, then a resistor must be used to drive the bjt. This is why bjt is "current driven". The Vbe is of course, absolutely needed, as are Ib & Ie. All 3 are needed for transisto action to occur. Without Ib, Vbe, & Ie, there can be no Ic. They are equally important.
Claude
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Kevin Aylward
6/2/2013 7:02 AM EDT
1 Semiconductor companies possibly just repeat the old wife’s tales that they know about. Possible its because good analog designers are hard to come by.
2 University semiconductor courses do indeed teach that the transistor is a voltage controlled device.
3 The Gummel-Poon charge controlled model expressly describes the transistor as a voltage controlled device. This is explained here:
http://www.kevinaylward.co.uk/ee/voltagecontrolledbipolar/voltagecontrolledbipolar.html
4 Driving a bipolar from a current source for biasing results in unpredictable biasing. This is explained here:
http://www.kevinaylward.co.uk/ee/bipolardesign3/bipolardesign3.html
5 Bipolors are easily driven from low impedance sources, e.g. Emitter resisters can be used when driving bipolars from large, voltage driven sources. Additionally, adding large resistors to the base circuit results in more noise, and lower bandwidth.
6 I could go on, but the original comments above are, essentially, incorrect.
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pcsalex
7/15/2011 9:47 AM EDT
by the way on figure 7B that is not a rearranged cascaded, but a differential amplifier!
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