designing a Differential Oscilloscope Preamplifier Question
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Posted by: laurence ®

08/28/2006, 09:51:26

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im designing a preamplifier, but my waveform is sweeping at around 2.5 volts, as the is a preamplifier it should be swinging around 0.0 volts. Does anyone know would to fix this problem? i am only using discrete components no ic's as this is one of the specs.

the specs are as follows:
1. Balanced inputs, so either input may be grounded and the signal applied to the other input.
2. Voltage gain = 100 ± 3%
3. Frequency response to be flat within 1 dB from 40 Hz to 15 MHz.
4. Input (single ended) impedance > 1 Kohms;, input capacitance < 5pF.
5. Output impedance < 100ohms;.
6. The amplifier should be capable of driving a 100ohms; load (external) up to 2 volts peak-peak without any visible sine wave distortion.

so what i have so far is a cascode amplifier connected to a Class AB output stage.

when i place a capacitor in between the amplifier and the class AB a few things happen
1) i get crossover distortion
2) frequency response cuts off too much and
3) the gain is lowered to like 600mV pk-to-pk as i need 2V pk-to-pk

this project is a university project im doing, so no real fancy work or dead set accuracy is need. the only thing i need to adhere to is the specs... as i indicated before i can get most of the specs right but it wasnt sweeping around zero and when i place a coupling capacitor frequency or gain is compensated. i tried this with the 2N3904 which i can get more frequency with but the same thing happens. im using multisim to simulate this circuit as i have to produce a pcb for this project and we only get one pcb run at uni as it would apparently cost the department a lot of money. so each students only gets one pcb. and the specs for the pcb is 100mm x 50mm. so back to my original question is there any other way to make it sweep around zero without using a capacitor?

i was going to post wat i have done, however the image was to big so i posted it on a third party site.
the site is www(dot)animex.com.au/preamp.jpg

thanks
Laurence







Modified by Administrator at Mon, Aug 28, 2006, 10:11:31


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Re: designing a Differential Oscilloscope Preamplifier
Re: designing a Differential Oscilloscope Preamplifier -- laurence Post Reply Top of thread Forum
Posted by: randykimball ®
Barney
08/28/2006, 23:19:47

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Do you have a meg ohm resistor between the pre amp out put and ground to drain off drift, static, and magnetic energy (radio wave) inputs?
...as I try to remember.. it has been a few years.. you may need this tiny load for stability.

Yep, a cap really screwed it up, didn't it?

I wish I could remember better, because my final test in an Instruments and Measurement class in 1974 was to draw a functioning single trace o-scope, complete with time base and gain switches. .. I received 4.0 in that course (the most allowed then) .. so.. I must have known, at one time...wink...
...now which door did I enter here from? it must have been that one, it is the only one...hummmm..

Good luck..





The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

Modified by randykimball at Mon, Aug 28, 2006, 23:32:41


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