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300MHZ 0 bias AMPLIFIER

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C&B

New Member
Hello,

I am a software engineer with basic electronic skills gone mad starting a project that appeared to be straight forward. Wrong!! Frustrating! I have however turned my frustration into a goal, that being to learn something by taking the following described project to completion. If this project is appropriate to this forum let me know. I look forward working with you all.

My project is to replace a heavily biased, heat generating, hybrid amplifier with a 0 biased amplifier. The requirements for the replacement amplifier are: operating frequency 300 MHz max; input level 0-5v; output supply voltage 100vdc; 0 bias push-pull output stage; output drive level 10-90vdc; tunable gain 20-40.

My concept is to drive a push-pull pair with an op amp incorporating feedback to keep one of the output pair in conduction eliminating crossover distortion.

Is this concept viable?
 

crutschow

Well-Known Member
Most Helpful Member
Do you mean an output drive level of 10-90vac? Into what load?

You certainly picked a tough design for your first. Designing an amplifier to output 90vac at an RF frequency of 300MHz is a daunting task and not well suited to a beginner with just basic electronic skills. Designing with 300MHz op amps requires expertise in high frequency design (oscillation is just about guaranteed for a first design). RF layout is required and decoupling is critical. And a 90V, 300MHz output stage is pushing the ability of any available transistors to do.

I hate to be pessimistic but I would suggest you start with an easier first project if you want a reasonable chance of success. It's rather as if I, having some basic software skills, would try to build a DOS operating system for a microprocessor.
 
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Papabravo

Well-Known Member
Hello,

I am a software engineer with basic electronic skills gone mad starting a project that appeared to be straight forward. Wrong!! Frustrating! I have however turned my frustration into a goal, that being to learn something by taking the following described project to completion. If this project is appropriate to this forum let me know. I look forward working with you all.

My project is to replace a heavily biased, heat generating, hybrid amplifier with a 0 biased amplifier. The requirements for the replacement amplifier are: operating frequency 300 MHz max; input level 0-5v; output supply voltage 100vdc; 0 bias push-pull output stage; output drive level 10-90vdc; tunable gain 20-40.

My concept is to drive a push-pull pair with an op amp incorporating feedback to keep one of the output pair in conduction eliminating crossover distortion.

Is this concept viable?
It is not!
You need proper RF design tools
You need proper RF layout tools
You need a bench full of test equipment to characterize the active devices you intend to use.

What is with the 0-5V Input? Sounds like a logic level.

This is the equivalent of coding an OS or a compiler with hex digits and entering those digits with switches and lights!
 
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C&B

New Member
Do you mean an output drive level of 10-90vac? Into what load?

You certainly picked a tough design for your first. Designing an amplifier to output 90vac at an RF frequency of 300MHz is a daunting task and not well suited to a beginner with just basic electronic skills. Designing with 300MHz op amps requires expertise in high frequency design (oscillation is just about guaranteed for a first design). RF layout is required and decoupling is critical. And a 90V, 300MHz output stage is pushing the ability of any available transistors to do.

I hate to be pessimistic but I would suggest you start with an easier first project if you want a reasonable chance of success. It's rather as if I, having some basic software skills, would try to build a DOS operating system for a microprocessor.
Sorry for the poor terminology, what I was attempting to convey is the output in not required to go rail to rail to drive the load (cathode) whose load characteristics will have to be derived; I have not been able to find the data sheet for the CRT. Schematics and parts list are available for the existing circuitry. The maximum allowable current is 4ma; determined by analyzing an over current protection circuit in series with the load.
I have done a substantial amount of reading on the topic of RF amplifier design and have a good overall understanding of the issues you mention however without practical experience it is difficult to design around them.

Here is a concept drawing, consider this a block diagram only showing functionality:



See attached;




The concept is for the op amp to keep one of the two transistors in conduction based on the feedback. I based this on studying the workings of a voltage follower circuit. As you can see there are a few basic problems to overcome, mainly level shifting between the op amp and the transistor output stage. There is a catch I should mention; the load of the feedback circuit is restricted to micro amp range due to the method the existing circuit uses to monitor the load current.

Let me ask; is there a standard method to interface a low voltage op amp to a high voltage driver stage?
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
You're still giving no clue as to what you're trying to do, but if (as suggested) it's a scope Y amplifier - it's an incredibly difficult thing to design, particularly for such an excessive bandwidth.

I've also no idea what you mean by 'zero biased'?.
 

C&B

New Member
The original amplifier is a hybrid video pack, see attached, used to drive a high quality CRT projection device. There are two video packs used, one to drive the grid the other the cathode. These amplifiers are in conduction at all times (idle current) generating a significant amount of heat (estimate is 20 watts dissipation each). My effort is to design an amplifier that does not require significant idle current (micro amps). This will solve two problems, eliminating the unwanted heat and allowing the monitoring of the cathode cutoff current through the emitter circuit.

So, my definition of “0 bias” is insignificant current flow in the output stage emitter circuit during absence of input signal.

The attached drawing “amp01” gives an overview of my design keeping in mind the problem of interfacing the op amp voltage to the driver stage voltage is not addressed. Let’s assume the voltage to the op amp equals the voltage of the driver stage (100v) for this discussion.

A definition of terms:
V1 = video input signal
V2 = output stage drive signal
V3 = cathode signal
V4 = feedback signal
K = cathode
Imax = CRT cutoff current

The idea is to keep V3 = V1 x gain. At a steady state input voltage at V1 I would expect the voltage at V2 to oscillate +- .7v around V3 when making corrections to satisfy the input to the op amp V1 must = V4.
 

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C&B

New Member
Yes, that is the reasoning for the feed back to the high gain op amp. I see the op amp working like this:

1) if + input = - input than output = input - both transistors not conducting
2 )if + input < - input than output full low because of open loop gain - sink transistor in conduction
3) if + input > - input than output full high because of open loop gain- source transistor in conduction.



Is my logic faulty? If so, pleas explain.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
So you're expecting high gain from an opamp at 300MHz, have you even found one that approaches anything like that?.

Trying to overcome crossover distortion in this way also isn't very successful, but may be enough for a video application? - I don't know what effect it might have, I've never seen it done.
 

C&B

New Member
So you're expecting high gain from an opamp at 300MHz, have you even found one that approaches anything like that?.

Trying to overcome crossover distortion in this way also isn't very successful, but may be enough for a video application? - I don't know what effect it might have, I've never seen it done.
Your point well understood and admittedly I am having difficulty understanding how to equate the device parameters and graphs to my circuit design.

I am considering the AD8009 for this application because of the normalized gain vs frequency curves. I would appreciate your opinion on the use of this device in the amp design.
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
Looks a pretty good opamp, but it can only manage a gain of ten at 320MHz - you also need to be aware that your proposed amplifier scheme will only provide slightly less than +/-5V output, as the transistors are only emitter followers, and provide no voltage gain. So the output can only be the same as the opamp output, which is specced for a +/-5V supply.
 

C&B

New Member
It is not!

This is the equivalent of coding an OS or a compiler with hex digits and entering those digits with switches and lights!
Thanks for that comment, it brought back old memories and gave me a chuckle!

Speaking of tools, what tools are available here on the forum for simulation of analog circuits and what type of file is needed? I have access to a schematic program PCad
 

C&B

New Member
Looks a pretty good opamp, but it can only manage a gain of ten at 320MHz - you also need to be aware that your proposed amplifier scheme will only provide slightly less than +/-5V output, as the transistors are only emitter followers, and provide no voltage gain. So the output can only be the same as the opamp output, which is specced for a +/-5V supply.
Ok, it looks like we are on the same page regarding the concept. So the real question is how to interface the low voltage opamp to the high voltage source with a gain factor?

This is what I consider to be the tricky part of this design! I have spent a lot of time thinking about it with no good concept to date. Some ideas / techniques with this will really help!
 

Papabravo

Well-Known Member
Thanks for that comment, it brought back old memories and gave me a chuckle!

Speaking of tools, what tools are available here on the forum for simulation of analog circuits and what type of file is needed? I have access to a schematic program PCad
Many people are fond of LTSpice (aka SwitcherCad) from Linear Technology. The package is well maintained and has an active Yahoo group with many knowledgeable analog designers.

Linear Technology - LTspice IV Downloads and Updates
 

Hero999

Banned
The requirements for the replacement amplifier are: operating frequency 300 MHz max; input level 0-5v; output supply voltage 100vdc; 0 bias push-pull output stage; output drive level 10-90vdc; tunable gain 20-40.
What, 300MHz at 90Vpp?

You'll need a TWT or klystron to get that kind of bandwidth at that voltage.
 

C&B

New Member
I would like to post a schematic snippet from the service manual for reference purposes.

Is it allowable on this forum?
 

unclejed613

Well-Known Member
Most Helpful Member
try LTSpice available for free from Linear Technology - Linear Home Page

it draws schematics and simulates the operation of the circuit.

the reason those video amps are dissipating so much heat is that they're operating in class A. a class A amplifier is always in conduction and so dissipates a lot of heat. sounds like you're trying to eliminate the quiescent current, which means using class B operation or class C operation. the problem you will experience is that class B and class C operation is very nonlinear. if the device in question is a color monitor, any nonlinearities will be very evident in the picture. what you are describing sounds like a kind of a push-pull class C amp, which is fine if your drive signal is digital, on or off, but if you're displaying analog video, it won't work. if you're displaying analog video, the quiescent current and the heat are neccesary evils.
 
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