noise problem in voltage booster circuit

excuse me noise level at point D is about Zero;

Well, you're going to get noise there because transistors in general make noise at certain regions. Thermoshot noise comes to mind also!, you're using a 741 which isn't the cleanest Op-amp there is. use a small capacitor to ground on the Base at C and Collect at B,. also, a small choke in series with the collector then cap to common helps. some where in the hood of .1uf should do it.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
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jsmith,

First of all, what is "vin"? DC? AC? Signal? What frequencies?

Also, what type is Q2? And what value/type is L3? And C54 and C55?

Some possible suggestions:

This appears to be an oscillator. I didn't have a 741 model to simulate it with. But, with 2V DC for vin, and an OP275 (and 500K for the 1M pot), it oscillated at about 180 kHz, after about 70 ms. And with an LT1007, it oscillated at about 55 kHz, after about 49 ms.

And if the bottom of the 1M pot is connected to the opamp's negative input, instead of to the positive input, it appears to just add 75 to

90V DC to the input signal (depending on the 1M pot's value), while attenuating an AC vin input signal by about 80%.

But, _assuming_ that you just drew the circuit's schematic incorrectly, for this post only:

Can you use a better opamp?

Can you lower the resistances' values?

Depending on what vin is and what it's for, you could try a capacitor from point B to GND.

- Tom Gootee

-

Thanks Jamie I before replaced 741 with OP07 but the result is the same. Also before there was a RC(1K,2n2) between collector and base that force circuit to oscilate, other values for C (20p to 100n) didnt produce good results. I will try with choke.

Thanks Tom

Vin is DC Q2 is KSP42 L3 680 uH plastic axial C54,55 are 1uF multilyer U10 is OP07

because of low output impedance of +100V supply source, I have to use this resistances values, but if you think change of resistors, produce considerable change in noise, I will replace +100V supply. I will try with change configuration of inputs of opamp and polarity of DC Vin. The cap between B and GND make circuit oscillate with this configuration.

(snip)

You might try adding a 1 nF cap (1000pF) between the opamp - input and its output, to roll the response off at about 1 kHz. This eliminates most of the frequency response that might contribute to oscillation.

Because of high voltage at B point I cant control PNP transistor with opamps with +-15V power supply, so the bottom of the 1M pot is connected to the opamp's posetive input istead of negative input with PNP transistor control

I ment between the Collector to Grd and another from base to grd.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Thanks John I will try it.

quoted text -

Dear Jamie if you ment, use both of them together, i will try it

I took a better look at your circuit, and it would be a lot better if you could use a lower impedance drive to the transistor than 330k and lower the voltage gain of the transistor stage with emitter degeneration.

For instance, if you added a 15k emitter resistor to the -15 volt supply, eliminate the base diode and 330k base resistor and replace them with a 12k resistor opamp output to base and 10k from base to -15 V supply, the stability and noise of the high voltage stage improves dramatically. This biasing allows the output to follow an input all the way down to zero volts. And you don't need any of that output filter stuff.

I would still use a high frequency roll off capacitor opamp output to - input, but it could be reduced to something like 22 to 47 pF to extend the closed loop response out to almost 10 kHz.

Thanks John I will try it.

What is the ripple voltage, now at the 100V supply, and what filter capacitance does it now have across it?

Thanks only 1nf feedback cap was good and decreased noise to 50mv. FFT of noise shows 100Hz peak noise harmonies. how can i calculate L(or LC) filter for decrease this noise (from +100v power supply)?

It is only the above +-50mv noise. parallel 220uF, 100nF, 10nF after bridge diode + LRC [680uH,

20K,parallel 100nF,10nF,1nF] before load

In that case, you need more loop gain to reduce the ripple with feedback. But your original circuit is not stable unless you reduce the loop gain with the capacitor I mentioned. However, the later version I recommended does have about 10 times more loop gain at 100 Hz and will reduce the ripple at the output about 10 times better. Here is a repost of what I recommend, with one improvement, a capacitor in parallel with the base drive resistor, to improve the phase margin:

If you added a 15k emitter resistor to the -15 volt supply, eliminate the base diode and 330k base resistor and replace them with a 12k resistor opamp output to base, paralleled with 100 pf and 10k from base to -15 V supply, the stability and noise of the high voltage stage improves dramatically.

I would still use a high frequency roll off capacitor opamp output to - input, but it could be reduced to something like 47 pF to extend the closed loop response out to almost 10 kHz.

You can also place a resistor in series with that capacitor to add a zero to the system. This lets you push the gain crossover higher than you otherwise can because it takes out some of the phase shift at higher frequencies. Normally, you would place the zero almost at the gain crossover frequency.

I would have to have the transistor type to figure out if that resistor would be helpful for not. As it is, it is cancelling the base capacitance, approximately, including the Miller capacitance.

Since I didn't specify the value, I maintain that it can be optimized for any transistor that may be there. If you had some mythical transistor with no capacitance, a quite large value would be used. For any real transistor, you can move the zero to line it up with a pole. I can't think of a case where zero would be the best value.