P3A AC calculations problem

[webald0]

With a great help from sir godfreyl i did my DC calculations. Now, i started doing AC and already have a problem

So, this is scheme of P3A amplifier which i am calculating ----> **broken link removed**

I have a problem in input filter of amplifier. My simulating program is Isis Proteus so i`ll put some pictures from it for better understanding.

Function generators voltage is 100mV (70.7 mV RMS) with frequency 1kHz. (Vcc and Vee are 30V, overall 60V, but i think it doesnt matter in this case)

With only input filter the voltage is 70.7mV over the resistor and thats ok. (first picture)



As soon i put resistor in parallel with filter, i dont know whats going on anymore. (second picture)



The problem is even bigger with capacitor in parallel with that resistor (oscillator?) (third picture)



I tried impedance calculations and many other things but i cant get results same as in simulation, so if somebody understand this better can you please explain it, or give me a clue and path to follow so i can get it myself.

Thank you very much.

 

[orbanp]

It is not clear what are you simulating.
Here is the simulation in LTspice, and it is working fine.
Suggestion is to go through some tutorials.

Good luck, Peter

 

[webald0]

Thank you for reply. It looks very nice in LT Spice, but you did advanced thing for me yet

my problem is to calculate currents and voltages in dynamic analysis. As you can see on my pictures, currents and voltages aren`t the normal voltage/impedance case, because of Vcc and Vee of Diff. Amp.

Sin voltage from function generator is 100 mV, and without Vee and Vcc i am getting normal values. But as soon as i put Vee and Vcc on 20V, i dont know how to calculate currents in filter and RC parallel circuit.

English is not my language so this could be very hard to understand, i hope you will. Ill take time and put all equations and screenshots here so you can understand it better

Thank you once again!!

 

[orbanp]

Hi WebaldO,

My apologies that you did not find the reply helpful, however, you might as well follow the advise, and go through some tutorials on electronics, it would serve you well! Simulation is a powerful tool only if you know how to interpret the results!

Some points.
Picture 1 is not a filter, it just an R and C component in series.
Picture 2 is still not a filter (as far as the P3A amp is considered), it is just a voltage divider with a DC blocking capacitor. If you push it, you can consider it as a high pass filter, its effect is shown on my simulation at the lower frequencies. The value shown at 1KHz is correct.
Picture 3 is the low pass filter (not an oscillator), that is what an audio amp usually has, it does not let high frequency components to the amp. In the simulations its effect is shown at the higher frequencies. Again the AC voltage shown in your measurement is correct, at 1KHz the filter does not attenuate the input signal.

Some other suggestions.
Initially use AC simulation that sweeps the frequency in the range you are interested in. Use a scope as a probe, a picture is worth a thousand words (voltage and current measurements). You will get a similar curve like mine.
When you are satisfied with the overall transfer function of the amplifier you can follow up with the transient analysis!

But the first thing, you really need to read up on electronics!

Peter

 

[webald0]

Why are you apologizing, you are doing me favor with your replies!

Thank you very much for your answer and suggestions. I`m really trying with all those tutorials but its not just piece of cake, this answers really help me.

What means push the RC? To put a signal through RC, then it works as a high pass filter?

In all this tutorials i have read it says that RC in parallel does not act as a filter unless driven by a current source.. That`s why i said "oscillator", it makes more sense if it is low pass filter.

- - - Updated - - -

Ok, after some thinking ---> Q3, R8, R7 and D1 are current source, so Q1 and Q2 collectors have same constant current. Also the base of Q1 and Q2 have constant currents, so actually there is a current source (or constant current) flowing through C2||R2 so it is a low pass filter. Am i right?

 

[orbanp]

Rereading the first reply, I was a bit terse, and not too helpful explaining what you were seeing there.

C1 R5 is in fact a high-pass RC filter, "pushing it" meaning that it was not the intended purpose, but to separate DC voltages from the input. As a side effect it also acts as a high-pass filter.

In real life you do not have ideal voltage and current sources, in practice a voltage source has an internal impedance, or a current source has an internal admittance. One can be recalculated into another.

You do not show any circuit diagram, Q1, Q2, etc. does not refer to anything!

Again, suggestion is to find an introductory text book on circuit analysis, and go through that book. Modern text books use examples using Spice!
Go to a local university book store to find such a book (they usually also sell used books).
This wood do wonders to your understanding of the problems!

Peter