Author Topic: common emitter biasing  (Read 6038 times)

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Online Ian.M

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Re: common emitter biasing
« Reply #25 on: June 11, 2015, 11:18:56 am »
If you are interested in a pulse output you may find the trailing edge of the pulse is degraded due the the transistor's recovery time from saturation.  If you need fast pulses, you may need a Baker clamp to keep the transistor out of hard saturation..
« Last Edit: June 12, 2015, 01:31:22 am by Ian.M »
 

Offline socratidion

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Re: common emitter biasing
« Reply #26 on: June 11, 2015, 11:55:29 am »
Aha, I am indeed finding the falling edge isn't as sharp as it could be. I shall look into your suggestion.
 

Offline T3sl4co1l

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Re: common emitter biasing
« Reply #27 on: June 12, 2015, 12:21:47 am »
If your signal source is something sharper already (like a square wave, but maybe not very good risetime), you can skip all the biasing junk and use a voltage divider from input to base to GND.  You design the voltage divider so that it has a base voltage of about 1.4V (assuming the base didn't act as a diode), and a Thevenin resistance of enough to deliver the required base current (usually, Ib = Ic / 20 or so).  This way, when the source is 'on', the base gets +Ib, holding the transistor in saturation (a 2N3904 will turn on in ~30ns, depending on various conditions).  When the source turns off, the base capacitance is discharged to GND, with about -Ib of current (a 2N3904 will probably turn off in 100ns under these conditions).

As I mentioned earlier, you can cascade stages to increase the gain and reduce the risetime, at least to a point.  A "switch" circuit like this is really just a linear amplifier, at some point of its swing -- it's just not intended to stay in that range very long, hence the lack of biasing!  After a few stages, if the risetime is now limited by the circuit itself (and not the signal source), you need to change up the circuit.  Usually, more -Ib, more Ic, and lower resistances is required to get there.  Under ideal conditions, you can get ~10ns edges out of a 2N3904.

Tim
« Last Edit: June 12, 2015, 12:24:24 am by T3sl4co1l »
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Offline socratidion

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Re: common emitter biasing
« Reply #28 on: June 12, 2015, 07:21:17 am »
A lot of information in the last few posts – it may take me some time to digest and/or try out. I very much appreciate the advice and technical help.
There are of course other ways of making pulse waves from other shapes (a comparator, yes; or a 40106, or indeed any logic gate IC wired the right way). I'm being methodical, trying to imagine every possibility, and seeing what comes out.
The input signal is mostly lopsided triangle shapes at the moment. I've noticed that changing the base bias gives me control over the pulse width of the output. It strikes me that (with a few things added) this would be a way of making symmetrical waves out of unsymmetrical ones, e.g. squares out of pulses, triangles out of saws. (There are other ways of doing this too).
 

Offline T3sl4co1l

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Re: common emitter biasing
« Reply #29 on: June 12, 2015, 08:52:18 am »
A single transistor is, of course, the simplest case of amplifier or comparator; all about how you use it.  Ignoring Vbe, if you apply voltage to base with a current limited voltage source, and to the emitter with a constant voltage source, then the collector current is either on or off based on the input voltages.

The best way to get a constant voltage source is to use an emitter follower, which gets you a differential amplifier topology (the conventional one if using the same polarity transistors, or a "complementary differential" if using opposite polarity -- which isn't very useful because you still have to mind base-to-base voltages, but comes up on rare occasion).  Sometimes you can arrange the circuit so you have a low impedance already, and get away with just one transistor.

Tim
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