Mosfet amplifier help

Thread Starter

coinmaster

Joined Dec 24, 2015
502
Hello again, I have a opamp servo that has a max output voltage of about 40v and I need to output more voltage than it's rated for. I need a linear voltage swing of +25v (maybe more) to -150v.
I'm starting to reach the point where my limited understanding of transistors is becoming a bottleneck.
From my understanding, mosfets are terrible voltage amplifiers and will need some sort of feedback to linearize them. Also from what I understand I would need a P type mosfet and they don't amplify positive voltages correct?
Any advice on how I would go about doing this?

To sum up I need to have my opamp servo which currently swings +/- 40v and amplify its output with a gain of 5 so it can become +/-150v which will be used as a voltage reference for a vacuum tube's grid bias.
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
I'll be ordering a custom transformer for this so the supply can be whatever I need it to be. I have bench supplies that can go up to 300v (800v if I really wanted) at the moment.
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
.05 miliseconds from the change in voltage on the inverting input of the opamp. It's to keep 0vDC null on the output of an amplifier and it will be applied to the grid of a tube where 20hz-20khz is applied. Then again I wonder if it matters, I'm not sure if a change in voltage as fast as the AC frequency would end up canceling out the signal, I think the better answer would be I don't know.
 
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crutschow

Joined Mar 14, 2008
24,055
.05 miliseconds from the change in voltage on the inverting input of the opamp. It's to keep 0vDC null on the output of an amplifier and it will be applied to the grid of a tube where 20hz-20khz is applied. Then again I wonder if it matters, I'm not sure if a change in voltage as fast as the AC frequency would end up canceling out the signal, I think the better answer would be I don't know.
If you want a DC null on the output then you don't want to respond to the AC voltage, just it's average (DC) level.
So I would think you would want its response to be much less than 20Hz, probably a fraction of a Hz, perhaps with a integrator in the loop so there is no steady-state DC voltage.
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
1hz is 1 second. I don't want a 1 second lag for my DC null, that will cause a mess of problems in my design. How fast does an opamp switch voltages?
 
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Thread Starter

coinmaster

Joined Dec 24, 2015
502
If you want a DC null on the output then you don't want to respond to the AC voltage, just it's average (DC) level.
So I would think you would want its response to be much less than 20Hz, probably a fraction of a Hz, perhaps with a integrator in the loop so there is no steady-state DC voltage.
I think you are trying to help me get my servo to work. My servo works fine all I need is to up the output voltage. Surely someone knows how to get 5x gain out of a mosfet?
 

crutschow

Joined Mar 14, 2008
24,055
1hz is 1 second. I don't want a 1 second lag for my DC null, that will cause a mess of problems in my design. How fast does an opamp switch voltages?
You say you don't know how fast you want it to adjust the voltage and then you say a 1 second lag is too slow.
How will you keep a faster adjustment from affecting the low frequency response of the amp?

And you can bootstrap DC as dannyf suggested.
Here's an op amp bootstrap approach for a high voltage output.
For ±150V output the transistors should be rated for at least 200V.
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
You say you don't know how fast you want it to adjust the voltage and then you say a 1 second lag is too slow.
How will you keep a faster adjustment from affecting the low frequency response of the amp?
I don't know the exact speed but the servo functions fine as is, I've tested it in my amplifier. Perhaps you know something I don't?
servo-1.png
It's simplified but you get the idea, the opamp takes its input from the output of the amp and then biases the triode until the output is zero. Is the opamps speed not near instant? Does that 1uf capacitor slow it down?
I'll take a look at that link, thanks.
 

crutschow

Joined Mar 14, 2008
24,055
That circuit is an integrator with a time-constant of 100kΩ * 1μF = 0.1s, giving an integrator unity gain frequency of 1.6Hz.
The output thus does not change instantly.
What does the signal input circuit to the grid look like? What signal is driving the grid?
That, along with the integrator response, will determine the amps low frequency response.
 
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Thread Starter

coinmaster

Joined Dec 24, 2015
502
The input signal is from a triode with a MU of 20 which probably makes a 20v rms AC signal based on typical dac output voltages which goes through a 1uf coupling cap to grid.
 

crutschow

Joined Mar 14, 2008
24,055
The input signal is from a triode with a MU of 20 which probably makes a 20v rms AC signal based on typical dac output voltages which goes through a 1uf coupling cap to grid.
Do you know what the output resistance of the triode is?
Also what is the gain from the grid to the output load?
 
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