High Voltage Op-Amps

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Frosty_47

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Hello,

There seems to be a deficiency in operational amplifiers with output voltage swings above 100V (OPA454). High Voltage op-amps made by APEX such as PA89 can be biased by + or - 600V, with similar output swings, however their $680 price per unit will shock most designers. National Semiconductor provides a high voltage op-amp (LME49811 and LME49830) that can be operated with voltages up to + or - 100V (200V differential), however the output voltage swing is limited to about 68V rms when biased with + and - 100V. This may not be enough for certain applications. So I would like to ask if it is possible to achieve higher output voltage by using single supply to the LME49811 (for instance: V+ =200V and V- = GND). The datasheet does not seem to mention anything on this matter.

Thanks

Note: Assume the input signal to the op-amp is TTL (above GND level).
 
It's possible to add transistors to boost an op-amp's usable voltage.

I don't have the circuit handy but many audio amplifiers use this principle.
 
Hero999 said:
It's possible to add transistors to boost an op-amp's usable voltage.

I don't have the circuit handy but many audio amplifiers use this principle.
Click to expand...

Yes I know that I can use common-emitter amplifier however, my load is varied thus I cannot achive stable voltage gain using BJT's. I need a very stable output voltage which can be achieved through the use of op-amp.

Oh, and I totaly support that https://www.stop-microsoft.org
 
I suppose now wwould be an appropriate time to ask you what you are trying to do with these op amps (WHAT you're trying to do, not HOW you're trying to do it though that would help, it is only of secondary importance)- perhaps you are going about it the wrong way and thus not able to find the parts you need as often seems to be the case.
 
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Sorry can't tell ya, it's top secret. But since, you've been here for a while, you can probably be trusted. In any case, the 100 megaton Frosty-M carrier guided (by the currently in design process) Ultra-high frequency navigation system is indeed a secondary importance, as the primary importance in this case would be: "Hmmmm... I wonder where that thing is going...". Hence I need a stable high voltage source to drive the piezzo crystal.

Anyways, I am trying to boost a small signal source to 120V rms. This will be done is parts. That is, the signal will be equally split into two amplifiers. The first amp will boost the positive half-cycle, while the other will boost the negative half-cycle. The signals will than be recombined and current-boosted to around 12A.
 
Frosty_47 said:
Yes I know that I can use common-emitter amplifier however, my load is varied thus I cannot achive stable voltage gain using BJT's. I need a very stable output voltage which can be achieved through the use of op-amp.
Click to expand...


You may use:
>> a "normal" op amp, but without the ussual negative feedback
>> followed by pair of complementary common emiter amplifier (to achieve the desired high voltage),
>> eventially followed by a pair of complementary emiter follower transistors if you need high current
>> Feeding the transistor's output (thru a voltage divider to scale it to a lower voltage level, compatible with the op amp) to the op amp's feedback input.

This way the output transistors (both the voltage gain stage and the emiter followers) are inside the op amp feedback loop and their gain is predictable.
 
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How did you determine that you will need 12A? That sounds excessive.
 


Thank you for you suggestion. I don't see how this will work because there is no DC component in the output of the BJT because it is capacitively coupled. So even if I feed a portion of the output back to the op-amp, where is the bias current (Ib) going to come from ? Without proper bias current at both op-amp input terminals, the op-amp will not work.
 
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I don't believe anyone mentioned a capacitor at all?, why did you mysteriously add one?.

But as suggested by others, your requirements are still too vague - we've no idea what you actually need, or what the signal might be.
 
Nigel Goodwin said:
I don't believe anyone mentioned a capacitor at all?, why did you mysteriously add one?.

But as suggested by others, your requirements are still too vague - we've no idea what you actually need, or what the signal might be.
Click to expand...

Originally I was going to use a single polarity supply to the transistors. That's why the coupling caps needed to be incorporated. However, I decided to use dual polarity and a Darlington-pair CLASS AB amplifier. This way I won't need the coupling capacitors since the output voltage rail will be around 0V lvl (assuming components are properly matched and heatsinked). However, I still don't see how feeding a portion of the output back to the op-amp will work because the 0V DC lvl will not provide sufficient biasing current. I tested the 120V boost circuit found in this guide: https://www.electro-tech-online.com/custompdfs/2008/09/AN-272-2.pdf

It did not work in MULTISIM. All I get is DC rail of 0V at the output (just as I thought would happen).

P.S. I need to boost a 2.5Vp sinewave signal to 115V rms. Oh and no, I don't want to incorporate a step-up transformer into the design...
 
Frosty_47 said:
Oh, and I totaly support that https://www.stop-microsoft.org
Click to expand...
Have you had a bad experiance with Microsoft?

If so then please sign up and post.

Feeding the output back into the input on an inverting amplifier creates negative feedback which results in a predictable gain, increased bandwidth and reduced noise.

It did not work in MULTISIM. All I get is DC rail of 0V at the output (just as I thought would happen).
Click to expand...

Which circuit did you build?

I assume you're talking about figure 6?

I don't see why it shouldn't work.

Have you used the correct component values and not done anything daft like not connecting all 0V nodes or using a single 120V supply rather than a positive and negative 120V supply or forgetting the positive and negative 15V supply?

P.S. I need to boost a 2.5Vp sinewave signal to 115V rms. Oh and no, I don't want to incorporate a step-up transformer into the design...
Click to expand...

The circuit will give a gain of -Rf/Rin = -100k/10k = -10 that's right the formula is the same as the usual inverting op-amp formula.

The booster circuit is quite easy to understand.

Q1 and Q2 form a common base amplifier with a low input impedance.

Q3 and Q4 form a common emitter amplifier with a high gain and higher input impedance.

Q5 and the diodes forms a current regulator that biases Q6 (a high gain common emitter amplifier).

Q7 and Q8 form an emitter follower with a high input impedance and very low putput impedance.

The buffer is placed in the useual feedback loop.

The booster is an inverting amplifier so the usual input connections are reversed.

The 20pF and 100pF capacitors added to the feedback loop act as a phase shift compensation network which prevents oscillation.

To increase the output current:

Q7 and Q8 need to be darlington transistors.

The 100R resistors need to be reduced to 4R7.

Two more diodes need to be added to Q7 and Q8's bases.

The supply voltage to the circuit needs to be a minimum of 170V to get 115VAC RMS on the output.

It's possible to power this circuit from a single power supply, but the input volatage needs to be 340V to get 115VAC RMS on the output.

Is 1.2A the peak current or RMS current?

If it's the latter, then you'll need to increase the suppy to >175V.
 
Hero999 said:
Have you had a bad experiance with Microsoft?

If so then please sign up and post.
Click to expand...

Lolz If I were to post all the problems with Microsoft, I would probably never stop typing as usually there will be problems encountered on the way

As for the circuit, thanks for explaining it. I did duplicate the circuit in multisim and checked all the connections. Input signal and load were also identical to that of figure 6.

One thing confuses me though, how does the 27K resistor in figure 6, provide any bias current to the inverting terminal when it's junctions are at GND level? (earth GND and virtual GND that is). Oh and I am really confused by the positive feedback in Op-Amps, can you please explain why the output is 180 degrees out of phase with the input in such configuration? After all, the input is applied to the non-inverting terminal so why would it be inverted? (Perhaps it's so due to common-emitter amplifier?)

Thanks!
 
Frosty_47 said:
Lolz If I were to post all the problems with Microsoft, I would probably never stop typing as usually there will be problems encountered on the way
Click to expand...
I still hope you sign up, we're a bit short of really active members who post loads.

As for the circuit, thanks for explaining it. I did duplicate the circuit in multisim and checked all the connections. Input signal and load were also identical to that of figure 6.
Click to expand...
Then perhaps you've made a mistake with the simulation settings/models.

Could you please post a scheenshot.

One thing confuses me though, how does the 27K resistor in figure 6, provide any bias current to the inverting terminal when it's junctions are at GND level?
Click to expand...
It just helps to compensate for the input bias currents.

It's normally equal to Rf and Rin in parallel so I would've used 9k1 but I think they've used a higher value because the output buffer boosts the voltage so much? I don't know why it's so high to be honest.

I explained that in my previous post, please re-read it.
 
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Hero999 said:
I still hope you sign up, we're a bit short of really active members who post loads.

Then perhaps you've made a mistake with the simulation settings/models.

Could you please post a scheenshot.

.
Click to expand...


Ok I signed up, here is a link to my first post:
https://www.stop-microsoft.org/bbs/index.php?topic=12082.new#new

As far as simulation is concerned, I will post a screen shot after I rebuild the circuit (didn't save sorry). This might take a while as I have no acess to internet from home (school time only).

Thank you for your help!
 
Great.

I like your signiture by the way.

Do you understand negative feedback in the booster now?

I'll try simulating it in LTSpice.
 
Hero999 said:
Great.

I like your signiture by the way.

Do you understand negative feedback in the booster now?

I'll try simulating it in LTSpice.
Click to expand...


Yes, seems clear at this point.

Thanks!

Oh, and please visit Most random videos in **** chat section
https://www.electro-tech-online.com/threads/most-random-videos.42638/

I am sure you will find some funny videos there...

Oh and there are a lot of quotes from Joseph Stalin. Some can be scary...

“Death solves all problems - no man, no problem.”

“A single death is a tragedy, a million is a statistic.”

“When we hang the capitalists they will sell us the rope” (LOLZ I LOVE THIS ONE!)

“If the opposition disarms, well and good. If it refuses to disarm, we shall disarm it ourselves.”

"Education is a weapon whose effects depend on who holds it in his hands and at whom it is aimed.”

"Mankind is divided into rich and poor, into property owners and exploited and to abstract oneself from this fundamental division and from the antagonism between poor and rich means abstracting oneself from fundamental facts”
 
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Hero999 said:
Q5 and the diodes forms a current regulator that biases Q6 (a high gain common emitter amplifier).
Click to expand...
Hero, I think you had a brain fart here. Q6 is an emitter follower with a voltage gain of slightly less than one.
 
You're right and Q5 is the constant current load for it.

I've just simulated it in LTSpice and it works perfectly.
 

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