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Variable gain and buffer circuit

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I am designing a PIC controlled signal generator. It can produce square, sine and triangle waves as well as PWM signals, RS232 signals, and simple mono and astable waveforms. The maximum output frequency is 25MHz, and the output voltage ranges from 0 - 20v peak to peak.

I am really struggling with the I/O buffer circuits and the variable gain circuit for the O/P. I have done quite a bit of digging on the web and have come across some buffer ICs, as well as digitally controlled potentiometers.

One of the buffer ICs I have found is the LMH6559 high speed, closed-loop buffer. Is it possible to introduce a feedback circuit with this IC to introduce gain for the O/P? Could the same buffer circuit be used for both the input and the output, without any gain for the input?

All of the digital potentiometers I have come across will only accept a maximum of +5v. If it is possible to include gain on a buffer IC this isn't a problem, but if it isn't does anyone know how to use one of these ICs in a gain stage? I am reluctant to use op-amps because of there limited bandwidth and slew rates, I am trying to keep my costs to a minimum.

Any information or suggests are most welcome. Thank you and merry Christmas!
 
Good luck with the extremely high speed buffer IC that will probably oscillate at a few hundred MHz.
I think you should use a high speed opamp instead then you can have the voltage gain that is needed.
 
I am thinking about a similar project at the moment.

Have you thought about using a 'potted' variable gain amplifier, such as the VCA822 ? This acts rather like an op-amp with the gain defined by a control voltage -1:ltoet: Vc:gtoet: +1, which can be easily generated by a DAC, voltage reference and an op-amp.
 
Audioguru,
Thank you for the reply. I was thinking about possible oscillations at the higher frequencies. I'm assuming I will be able to use the digital potentiometer within the feedback loop of the op-amp, even though it only accepts 5v.

cjb,
I've never heard of a potted variable gain amplifier. I will look into that, thank you for the suggestion.


Am I right in thinking that in order to match the O/P impedance of the amplifier to the bnc socket all I need do is connect a resistance of 50 or 75 ohms between the amp and socket?
 
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cjb said:
I am thinking about a similar project at the moment.

Have you thought about using a 'potted' variable gain amplifier, such as the VCA822 ? This acts rather like an op-amp with the gain defined by a control voltage -1:ltoet: Vc:gtoet: +1, which can be easily generated by a DAC, voltage reference and an op-amp.
I just took a quick look at the VCA822 datasheet, so I could be wrong, but it looks like a good part, except you can't get the necessary voltage swing. You would have to follow it with another op amp.The only one I found (in a quick search) that looks like it could handle a 20V square wave at 25MHz is THS3092/3096. This is a current feedback amplifier, and they are not easy to design in unless you have a lot of experience with wideband circuitry. I speak from experience. :D
I think a digital pot will have too much stray capacitance to handle 20MHz sine waves, let alone square waves.
 
Ron,

I too have now had a look at the potted amp idea, and to be honest, it all seemed a bit above me. It hadn't occurred to me that a digital pot wouldn't be able to handle the signals passing through it. However, would this be a problem if it was part of the feedback loop of a non-inverting amplifier?
 
canadianpoet2012 said:
Ron,

I too have now had a look at the potted amp idea, and to be honest, it all seemed a bit above me. It hadn't occurred to me that a digital pot wouldn't be able to handle the signals passing through it. However, would this be a problem if it was part of the feedback loop of a non-inverting amplifier?
I found a Spice model for AD5233 (a digital pot), and ran a sim with it as the adjustable gain element in a noninverting amplifier (I used a simple 2-pole behavioral op amp model). The amplifier oscillated due to the substantial capacitance in the pot.
I don't want to discourage you from learning, but I'm afraid you're way over your head here. I'm an old EE who has done high-speed analog design for 40 years, and this project would challenge me. Perhaps you should try something less difficult.
 
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