John Curl's Blowtorch preamplifier

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#18,421
scott wurcer said:


Are we looking at the same picture???? For infinite beta it's a diode connected transistor pulled up to the rail with a resistor (0V across the base resistor). Vce = Vbe. We don't consider that close to saturation.
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You forgot to mention in your question that beta is infinite. It is an artificial transistor. Also, it should have a zero saturation voltage, right?

So, are you looking for artificial employees for an artificial reality?
 
#18,425
stinius said:
Wavebourn

Why don’t you just stick to valves?
It seems like you are on very thin ice here and in the other thread.
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I stick to everything that is useful. Even to awfully non-linear thyristors. Such a way, facing reality as is, I can dance on thin ice creating desired results, instead of marching on painted roads only, repeating what had been done many times before.

dimitri said:
Scott, I see, saturation takes place with both junctions forward-biased, so if Vce = Vbe, Vbc=0 (zero bias).
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CE junction is not forward biased. When Beta is infinite Vcb is zero, in reality positive, so in any case it is not forward biased.

Edit: Oops... Sorry Dimitry, I thought you are still talking about Scott's test picture. Please disregard my answer.
 
#18,426
Wavebourn said:


You forgot to mention in your question that beta is infinite. It is an artificial transistor. Also, it should have a zero saturation voltage, right?

So, are you looking for artificial employees for an artificial reality?
Click to expand...

Wavebourn you seem to be laboring under the misconception that saturation is 0 Vcb this is certainly not correct. Transistors work just fine at .6V Vce. If you make the base resistor big like a meg you can get some volts at the collector but this is an unnecessary complication for a simple question. In fact I took the stock 2N3904 LTSpice model and ran it with 1Meg for the base resistor and 10k for the collector load 47.5dB gain for about 6 volts DC across the collector load even with 50 Ohms Ri. Pretty close to 240 or so.

And yes if someone does not know about gm they need not work in the IC business.

I used the LTSpice schematic because it was handy sorry for the confusion of the resistors being equal the base resistor should be at least 10 or 20x the collector resistor or there is a complication of dividing the output voltage and thus the apparent gain. This does not effect the thought process.

John should have seen 180 or so and got 160 no that far off.
 
#18,431
scott wurcer said:

Run transistor backwards Vsat = 0. Try it sometimes. Old Dick Burwen trick I think for making a bi-polar reset to ground that is identically ground.
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Did you see my posting in the other thread, about a negative voltage generation by NPN transistor when BE junction is reverse viased up to Zener breakdown?

I used Russian transistors KT208 long time ago. They were nearly reversible. They had 300 mV Vsat, but it was a long time ago. Modern Zetex devices have 20 mV, and hundred of Vcb breakdown volts.
 
#18,433
Scott, your example is a deliberately contrived one, based on virtually improbable conditions. That is the problem, but I do know what you are reaching for, and you are correct on this ONE point.
Actually, you are correct, that Re would most probably have to be added too, for stability, BUT adding the one base resistor to ground would make a more realistic circuit that you put forth.
 
#18,437
john curl said:
Just for thermal stability, but it also helps to increase the usable input impedance. In the old days, 1960, they usually bypassed Re with a big cap.
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So, they created feedback by current on DC, neutralizing it on AC by a cap.

Without your resistor from base to ground this shunted by cap resistor would be meaningless.

Here is the whole picture:


Originally posted by Wavebourn



Say, we power the amp from 24V.

A first, select a collector load resistor according to output resistance you want.

Now, assume a voltage drop on it will be 1/2 of Vs. Say, it will be 10K. That means about 1 mA.

Now, calculate an emitter resistor to get needed voltage gain. Say, for gain of 10 it will be 1K, 1V drop.

Now, calculate an upper resistor in the divider, according to minimal Beta (that is say 100), with lower resistor omitted: 1 MOhm roughly.

Now, make it 10 times lower for desired thermal stability and to make the stage less dependable on Beta: 100K.

Now, calculate the lower resistor for 1.5V voltage drop: 15K

What do we have?

We have a stage with voltage gain roughly -10.
10K in collector, 1K in emitter, 100K from collector to base, 15K from base to ground.

Very rough calculations, but now we can forget about Beta and it's variations from device to device, and with temperature. However, due to a parallel feedback by voltage it will show lower gain depending on the source output resistance.
Click to expand...
 
#18,439
Now, please, we invented the transistor, and GE was one of the first converts! I am taking the circuit from the 1960 'GE Transistor Manual'. You may have learned from a different source, but we still beat you to the Moon! Keep that in mind, when you confront me! :geezer:

Please don't 'quibble' (look it up), Wavebourn
 
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