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Old 11th October 2002, 01:32 AM   #1
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Default Biasing a Transistor

We all know that it is important to bias a transistor and it's some where near a mid point. But the question is in the data sheet, what is it we look for that determines how do we select the resistor to bias the transistor with real like transistors??? Take for example a PNP transistor BD140 or a NPN transistor BD 139. Also how do you know the Beta of the transistor from a data sheet.
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Old 11th October 2002, 04:03 AM   #2
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The beta is usually referred to as Hfe in the data sheets
for bipolar devices, and usually there is a curve of the
beta versus CE current. A typical value is around 100

For you Newbies:

Biasing a bipolar, or any other device for that matter,
depends on how you are going to use it. A bipolar is a
device whose CE current is a multiple of the BE current,
and there will be about a .65 or so Volt drop from Base
to Emitter.

There are three operating connections:

Common Emitter, with the signal going into the Base and
coming out amplified (and inverted) from the Collecter.

Common Collector, with the signal going into the Base and
coming out the Emitter (voltage follower)

Common Base, with the signal going in the Emitter and
coming out the Collector (often a cascode device).

You can establish a given amount of current through a
bipolar by injecting current into its Base from a high
impedance current source,

or you can put some finite resistance in series with the
emitter and feed voltage to the base,

or you can feed current into the emitter pin directly

Look at some simple circuits. It's a lot easier than you
might think.

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Old 11th October 2002, 05:11 AM   #3
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Hello Edo!

Don't forget that the current gain(Hfe, beta) of a bipolar device vary a lot from one sample to another.

A circuit that relys on a particular value of current-gain is a bad circiut. In my world



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Old 11th October 2002, 07:14 AM   #4
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Thanks Nelson for your intro in biasing.

I'm familiar with calculating from a diagram but i'm not really familiar when reading from a data sheet. For example from the data sheet given, the hfe or beta is 250 if i were to send a current of 150mv to collector. Thus using Ic/hfe=Ib, then to get hfe 250 i got to send a current of 0.6mA to the base to get 250 gain is that correct???

Morello, thanks for you advice. What then would you rely on if you were not to design a circuit from it's hfe. care to share some insight with a newbie??
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Old 11th October 2002, 07:56 AM   #5
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Hello!

Try to make the circuit rely on the values of passive parts such as resistors etc. If your bipolar device has a typicla current gain of 200. Contemplate what happens if it reduces to 100 or increased to 200. The manufacturing spread with respect to current gain is large.

Transconuctance in bipolar devices are a rather good parameter to rely on.

For small currents:

gm=Icq/25, Icq in millieamperes.
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Old 11th October 2002, 11:21 AM   #6
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Hi Edo,

A transistor is a highly intuitive device; a current injected or extracted into the base (depending on whether it is NPN or PNP) will control a current in the collector/emitter circuit according to a relationship called the beta, which you've worked out already.

The three configurations Nelson refers to determine how you will use the transistor - and the outcome. A common emitter, the most frequently used configuration, gives current and voltage gain, and makes the transistor work pretty hard, revealing, incidentally, all it's faults. A base current controls the collector current, which, if passed through a resistor at its collector - the load - gives a voltage gain in the same way as a tube. This voltage gain is given by the ratio of the collector impedance to the emitter impedance. If the emitter is grounded, then the emitter impedance is given typically by 26/mA, where mA is the current flowing through the collector/emitter (almost the same for most transistors since one is generally at least 99% of the other). When the collector load is in fact a constant current source, with very high impedance in the megohms range, then the ratio of these two is typically around 2000, or 66dB. This is a very high voltage gain, and must be throttled back for most practical applications. This leads naturally to global negative feedback, which serves precisely this purpose.

The next most common configuration is the common collector, sometimes (indeed often!) called the emitter follower. Here the collector is connected to a power rail, and the signal injected at the base from a high impedance source. The output is taken from the emitter, current (but not voltage) amplified. This is effectively an impedance transformer, and a fully complementary double emitter follower (two acting in cascade) is frequently used for current amplification in the output stage of an audio amplifier - and is roughly analogous to the differential on the wheels of an automobile.

The least used configuration is the common base, where signal is injected into the emitter at low impedance and taken from the collector, much amplified in voltage, but with no current amplification. This is of lesser use in audio, though still seen in the cascode, but has great advantages in bandwidth and is commonly used in radio frequency applications.

The way in which the base is driven in common collector and common emitter is difficult to describe, since a combination of both current and voltage is involved. That is, the base is not strictly driven by a current generator or a voltage source; it is driven by a combination of the two. By this I mean that the current drive to a common emitter amplifier - a voltage amp with current gain - varies, and with it varies the voltage drive between base and emitter, though not by very much, typically 0.6 volts to 0.75 volts. It is the mathematical combination of these two which gives us the transfer function of the transistor; a thing of some complexity, but not something you need to understand intimately in order to design with these elements.

You should know that beta, the ratio of collector to base current, varies with a number of factors; collector current, temperature, collector/emitter voltage, and even frequency. Because the resulting relationships are so complex, and so non-linear, we usually finish up loading the device with a very high impedance, such as a constant current source, and then fixing the overall gain by use of a negative feedback network, which absorbs all these non-linearities into a couple of fixed, accurate resistors which establish the overall gain of the circuit on a global basis, removing all the quirks of the transfer function. This works well enough, but care must be taken to bring overall gain below unity at a frequency where the negative feedback abruptly swings to positive feedback. We have to do this because positive feedback creates ever larger outputs from tiny inputs, creating oscillation, which can destroy our creation, and particularly our loudspeakers.

This is a complex area, but the basic rules rely simply on Ohms Law and a bit of common sense. It's a fascinating hobby, full of unexpected discoveries, and I do hope this is helpful!

Cheers,

Hugh

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Old 11th October 2002, 11:22 AM   #7
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Morello is right: the beta varies all over the place.
With bipolars the parameters vary alot from one device to the next, even though they have the same part number. A side-effect of making small things in silicon. So in the datasheet the beta is shown to vary from 25 to 250 depending upon device and operating point (a factor of 10!).

Other parameters vary too: Rpi, Cbe, Ccb, Cce, Ft. In the case of beta and Rpi and Ccb - these are most influenced by Ice and temperature. Ccb is most influenced by Vcb. If Vce drops much below a volt the transistor parameters start changing rapidly.

Morello is right to advise designing a circuit to be tolerant of beta variations, unless you are hand-picking the part and keeping its temperature steady during operation.

The small-signal transconductance is the Ice/Vbe and usually follows the Ice/25 rule closely for small-signal devices. Eg: the differential transconductance of a long-tailed pair is calculated from the gm of the two transistors.
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Old 11th October 2002, 06:07 PM   #8
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Beta and Vbe are also dependent on temperature,
so don't assume they are constant just because you
tested them.
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Old 12th October 2002, 05:05 AM   #9
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Thnkas for the insight on beta. But beta affects the close loop gain, the open loop gains and the unloaded differential right. But how does one find all these value??? are they in the data sheet as well??
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Old 12th October 2002, 06:07 AM   #10
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Quote:
Originally posted by Edo
Thnkas for the insight on beta. But beta affects the close loop gain, the open loop gains and the unloaded differential right. But how does one find all these value??? are they in the data sheet as well??
You can find beta(current gain) fram data sheets.

The gain in i particular gainstage depends on how you implement the transistor.

The voltage gain in a diff-pair(long tailed pair) os approximately:

Gain=Rc/( 2*Re + 2/gm)

Rc=collecor load

Re=emitter degeneration resistors

gm=transconductance=Icq/25

Icq=quicent current measured in milliamps

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