ВЭФ 12 (Astrad VEF 12 Project) - including a transistor tutorial

[Nowhere-Man]

I think what would be useful here is if somebody asked, what do we know about the current flowing through each part of a series of resistors or bulbs, in a simple circuit. How can we use that to work out the voltage across the 4.7K resistor, and deduce the volts at the collector of the transistor.

Mike

I was going to suggest that before but was wary of causing offence. Take the output load resistor. If you removed the entire transistor part, you get another divider. Let's suppose all the resistances were pure power watts (filament glow). Then, you can split the voltages into dividers and work out currents at various parts of the circuit.
That is something I have done before, by the way. I have been known to do exam questions although pretty much all of those were from a 1960 textbook. Kerchoffs law I have done before many months ago. As well as reactances, resonant frequency, time constants and so on. It's just (1) sometimes I have to refresh and (2) semiconductors is new terrain.
Still, once the transistor is in circuit, current will be amplified. My AARL book states this is due to much higher resistances at the PNP collector/base junction due to back bias so, therefore, the current multiplied by the very high ohms value creates a huge increase in watts. Basically all the angles are good so I'll bear with Mark and use textbooks as well.

[pwdrive]

At this stage I think that Mark and possibly others just want to know from you is what the DC collector voltage is in the circuit that Mark posted.

Lawrence.

[Nowhere-Man]

I will get back to that later. I am right now trying to place an order for transistors in Russia. Exchange rouble is 0.017 US Dollar so that figure times 10 is the price I can hopefully get for each germanium transistor. I won't be doing any theory till late tonight. The valves are also dirt cheap Russian pentodes but need more info.

[Crackle]

Do RL and RE form a simple potential divider, could the transistor junctions with the C to E connections have any bearing on the circuit.
It may be an idea to view it as similar to 3 resistors in series from the point of view Mark is trying to make.

Mike

[Mark Hennessy]

Earlier I linked to The Art of Electronics, and specifically "Transistor Man". I have no idea if you've had a chance to look this up - it's a big download - so here's a grab of the image I wanted you to look at. It might help here.

   

As Mike says, you may wish to think of the collector-emitter junction as a resistor of sorts.

Therefore, everything we have done so far has been to set the collector current to 1mA.

The final step of the journey is to determine the collector voltage.

This again is just the application of Ohm's law. But, because I'm very fair, I'll mention that there is a trap that folk usually fall into at this stage. Only a small one, but useful from the learning point of view, so don't feel bad if it happens... So I'll say no more at this point - over to you: what is the collector voltage?

Don't be afraid - this is simpler than you think. Heck, you don't even need a calculator...

[Murphyv310]

Hi.
Shoot me down in flames if you wish but the question is simple and has been said no need for calculators or textbooks, it should take all but a few seconds to come up with an answer.
My worry is as has always been is without even the grasp of simple things like this is how on earth can you even try to understand the workings of a frequency changer, or even the biassing arrangements of a valve or transistor amp.
This question could have been answered far quicker than ordering spares!!!
Please take on board what has been said and devote time to reply to those that have spent the time to try and help you.

[Crackle]

Or ask for the solution to be explained fully.

Mike

[Murphyv310]

Yes as Mike says.... just ask, we all have had to learn this at one time and no one will ever refuse help when its asked.

[Nowhere-Man]

"So I'll say no more at this point - over to you: what is the collector voltage?"

I don't want to hog this thread and am content to let others chip in a bit more but it's good of you to take the time so....
You explained that the emitter current added to the very small base current will equal the collector current. You quoted a the 1K resistance and by division of it into the 1 volt we agreed at the emitter lead, we wound up with 1 milliamp. I will write is as 0.001 amp. We ignore the base current (my ARRL gives in the region of 0.000125 amp).
What's the collector voltage? I am going on 4K7 as 4700. As it's the load resistor for the collector I'd say multiply 4700 by 0.001 to give 4.7. However, with a valve anode you would just use the current tables to drop whatever voltage difference exists between HT and listed anode voltage.
There was a trap? I did notice the 47 K and 4K7 could be viewed as parallel. Especially without the transistor. 4272 R would round out. Just out of curiosity I derived 0.001914893 - 0.000191489 and the deducted total is close to 1 milliamp at the collector.

[Nowhere-Man]

Please excuse any errors as it's late and I had a busy day. No time to proof read my summary so now going to watch a movie.