BJT dynamics

Thread Starter

roxi60

Joined Sep 2, 2018
73
Hello.
In enclosed file there's a BJT with a sine signal on base, through simply a capacitor.
I don't understand the reason why if I put no resistor from base to ground, I can't saturate the BJT, whereas I get it saturated if I put the resistor.
Why the BJT conduct better with the resistor from base to ground?
It seems that the base current is higher, but I don't understand physically why.

Thank you for any kind explanation.
Regards
Mod: extracted files from PDF/docx.E
 

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Thread Starter

roxi60

Joined Sep 2, 2018
73
Thank you Eric,Wolframore.

What Eric shows is quite the same signal on collector, but in Proteus or real circuit on breadboard is not so: in real circuit I have constant 8.1V on collector, when no resistance is between base and ground.
I would only know why, what does the added resistor do on the base current and voltage, that make the BJT conduct in one case and no conduction in the other?

Thank you.
 

Ian0

Joined Aug 7, 2020
1,129
Shouldn't the capacitor charge up and stop it switching? It can charge through the B-E diode, but where's the discharge path?
It should do the transistor equivalent of blocking distortion.
 

crutschow

Joined Mar 14, 2008
25,998
what does the added resistor do on the base current and voltage, that make the BJT conduct in one case and no conduction in the other?
It provides a path for the capacitor to recover the charge it used when the base-emitter junction is forward biased at the peak of the positive input.
It should do the transistor equivalent of blocking distortion.
I agree that makes no sense.
What "distortion"?
 

Thread Starter

roxi60

Joined Sep 2, 2018
73
Thank you all for your kind replies.
I just want to explain that the simple example I asked for is part of circuit you see enclosed.
Here a hand clapping make a sound that is amplified by a two stage (or not really a two stage) amplifier, the signal operates a monostable and jk flip flop and endly the power stage of the motor.
Here the sound amplified by the first stage Q1, is an impulsive signal that saturates the second BJT.
I know that the second stage is not biased, so can not be treated by a two stage amplifier, but I studied it alone and I was curious to understand why with only the capacitor C2 on the base, a sine signal can not saturate the Q2 transistor.
So the answer I'm looking for is:
With only the capacitor on the base (so not biased), the Vbe (or the Ib) is not enough to saturate the Q2 BJT because ... (fill in the blank).
Adding the resistor between the base and ground (or putting a probe of oscilloscope) the Vbe (or the Ib) is enough to saturate the Q2 BJT because ... (fill in the blank).
Sorry for the silly question, but I don't understand what happens in one case that lower the Ib.
Thank you again.
 

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Ian0

Joined Aug 7, 2020
1,129
I agree that makes no sense.
What "distortion"?
Blocking distortion - obviously something you've not heard of. Perhaps it's called something different in your part of the world.
Anyway - it's a thermionic valve phenomenon. (I can post the explanation from Merlin Blencowe's book if you want).
If there is a signal that overload a capacitively coupled amplification stage and forward biases the grid-cathode diode then the capacitor charges up rapidly, and cannot discharge through the grid-cathode resistor which is generally in the order of megohms. The charged-up capacitor stops the stage from working until it recovers, giving a few tens of milliseconds of missing signal.
I would expect the same thing to occur here especially in the stage without the base-emitter resistor. The capacitor will charge through the base-emitter diode to Vpeak-Vdiode. (about 0.3V), but cannot discharge. On the next cycle, the capacitor voltage will be subtracted from the signal voltage and there may then be insufficient voltage to turn the transistor on.
 

crutschow

Joined Mar 14, 2008
25,998
Blocking distortion - obviously something you've not heard of. Perhaps it's called something different in your part of the world.
No, I've never heard of that in my part of the world.
Why is that phenomenon called blocking distortion?
To me that means in stops distortion.
 

Ian0

Joined Aug 7, 2020
1,129
No, I've never heard of that in my part of the world.
Why is that phenomenon called blocking distortion?
To me that means in stops distortion.
I didn't name it!
Presumably because the voltage on the capacitor blocks the signal.
According to Mr. Blencowe it goes unnoticed until the amplifier input is overloaded - so it must be a phenomenon from the guitarist domain.
 

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Audioguru again

Joined Oct 21, 2019
2,380
When the input to C2 goes positive then the base-emitter current of Q2 charges C2 to the peak voltage (minus the base-emitter voltage) of the signal. Then when the signal goes negative the base-emitter of Q2 stops conducting which allows C2 to hold its charge. When the signal goes positive again then since C2 is charged then Q2 does not get enough base voltage to conduct again.

When you add a resistor to ground at the base of Q2 then it discharges C2 each time the signal goes negative.

I could not read the parts numbers because the parts on the schematic are so far apart that the text became tiny. So I cropped the schematic and enlarged it but the text still cannot be seen properly.
 

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Ian0

Joined Aug 7, 2020
1,129
Pretty much what I said in post #6 (before I realised that "blocking distortion" wasn't a widely-known term) and post #11; so why do BOTH waveforms back in post #2 show the transistor turning on and off?
 

Thread Starter

roxi60

Joined Sep 2, 2018
73
Please tell me if still unreadable the values.
But if you open the previous with no enlarging it should also be readable.

Thank you for all your kind comments and suggestions.
To fully understand I'm simulating the first post simple circuit as a clamper circuit, since B_E junction is a diode.
When there's the resistor, as you said, there's the discharge of the capacitor and the average signal of the clamper increases, probably enough to make the transistor conduct.
I'm trying to demostrate this with formula, but it's not easy also to find in literature examples of clamper with and without load resistance.
Thanks.
 

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