HV Amp - how does this amplify?

[keremcant]

hi,
I have this circuit that amplifies the signal from a DAC to a piezoelectric. So I need high voltages at the output, more than 50V.
The HV in the schematic is 115V.
The op-amp used is op27, which takes max +-22V as supply and the voltages that I see are 15.2V, -12V.
the npn transistor is mje340, pnp is mje350
the problem is, I guess, that the when both IRF9630 and IRF630 are open, the node piezo+ cannot decide where to stabilize, 115V or -15V, since both transistors are open.
So what is the use of these mosfets? Also what is the use of the bjts?
A little explanation would be great.
thanks.

 

[chuckey]

NO CIRCUIT So guesswork, the FETs are connected in series across the 115V line, the BJTs are meant to drive their gates so switching on the top FET gives 115V O/P, switching on the bottom FET gives 0V O/P.
Frank
On second thoughts if the HV supply was centre tapped, then it would be a better, it would keep the drive components closer to earth.

 

[keremcant]

ha ha,
sorry about that. the circuit is here:


your guess are mostly valid, but what I don't understand is howcome I can use the fet's as switches. because I want a sine wave as output for example, so I need a controlled voltage at the output, hovewer just opening and closing the fet's just pull the output up to 115V and down to -15V. Don't the fet's have to be controlled by the input signal in a way so that the output is dependant of the input signal?

thanks again

 

[keremcant]

HV Amplifier - biasing

Hi,
I have this circuit, a high voltage amplifier.
I've measured the bias voltages of the transistors and marked them on the schematic:

I have removed the connection of the drains of the fets. otherwise they pass too much current and burn, which make sense with the measured biasing values...
So what is my problem? What should be the appropriate values?
Also I still couldn't understand how this does amplify a signal, how can the switch-like fets allow a sine wave output?
thanks in advance.

Don't start new thread to ask questions about the same circuit. Threads merged [alexan_e]

 

[chuckey]

The BJTs have two functions :- 1, they are an emitter follower supplying the +- Vcc to the chip. 2, They are common base amplifiers pushing the change in the chips current into a higher voltage circuit which drives the FETs. There is something wrong with the lower transistor/-ve chip current 10.1,10.2,10.7 are wrong!
Frank

 

[keremcant]

There is something wrong with the lower transistor/-ve chip current 10.1,10.2,10.7 are wrong!

But the rest is right? Because the circuit is actually designed for HV = ~110V and the 20k res is actually 56k in the upper voltage divider part.

what should be the right values? what is the logic of the biasing? Any hyperlink that I can access to get some theory of the circuit would be great help.

What might this circuit be called? I google high voltage amp, power amp but I don't see any schematic that looks like this. Does this configuration have a name whatsoever?

thanks for your help.

 

[keremcant]

hi,
so should I bias the transistors so that their drain currents are equal? or should I try to arrange their Vds so that they can stabilize at a value at the output?
But I don't know the aspect ratios of the transistors, should I assume a value? I can empirically determine values too, but...
thanks?

 

[chuckey]

Going back to the static DC conditions, R9/R10 should give about -13.5V at J1's base and hence -12.9V at its emitter. This is the -ve VCC to the chip. the current taken by the chip flows via the 1.2K resistor which at present is limited to (18-10.2)/1.2K ~ 6.5 mA. The 10V is turning on U$7.
Try disconnecting the emitter of J1. if its working correctly, you should then get your 12.9V on it. Also the voltage drop across the 1.2K should fall to 0V.
Frank