preamplifier, need help!!

i have to design and build a self contained battery powered CRO balance preamplifier, to add high sensitivity ranges to an oscilloscope. This will be useful for testing audio and RF equipment up to over 15 MHz. The unit must be housed in a shielded box, that is, a metal or metal lined box.

Specifications
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 KΩ, input capacitance < 5pF.
5. Output impedance < 100Ω.
6. The amplifier should be capable of driving a 100Ω load (external) up to 2 volts peak-peak without any visible sine wave distortion.
7. Using this preamplifier on the 10mV/division CRO input range will boost the CRO sensitivity to 100 μV/division. Hence the project must be housed in a shielded box also containing the batteries, and care must be taken with the construction to avoid hum pickup, spurious oscillation, etc.

Components
The following components are allowed: (i) any passive components (ii) any discrete diodes (including zeners) or transistors, (iii) any voltage regulator ICs. No other ICs are allowed, including opamps.

i have used a differential amplifier connected to a cascade amplifier, i have done the simulations on multisim and i got the required specifications, however in real life its a different story, the main problem am getting is that my upper frequency response is no where near the required specs am getting about 300KHz but the required is 15MHz. Am currently using BC549 transistors. Any suggestions on how i could improve my upper frequency response will be greatly appreciated.

You forgot to post your schematic.

I have tried to post the schematic but when I try to attach the image a window pops up and asks for the URL of the image!! am not sure what that means, sorry am new in this website

Melissa said:

I have tried to post the schematic but when I try to attach the image a window pops up and asks for the URL of the image!! am not sure what that means, sorry am new in this website

Click to expand...

Click on "Manage Attachments" and then use the "Upload File from your Computer" window. You have to have the schematic stored somewhere on your hard drive, in one of the formats listed:
Valid file extensions: asc c doc gif h jpe jpeg jpg pdf png psd txt zip
I recommend .png or .gif.

Thanks for your help.

Your cascade is a good idea.
Your sim and your calculations didn't include the capacitance of the transistors and stray wiring capacitance creating a lowpass filter. The resistor values need to be reduced to increase the cutoff frequency and more feedback used in the output amplifier. Then the gain will be less requiring additional stages.

Look in Google at video amplifier circuits, you might find some old discrete designs.

You might also try placing smaller capacitors across the electrolytics, particularly the feedback decoupling capacitor - the electrolytics reactance will rise at higher frequencies and lower your gain, probably VERY substantially (as it's a high value electrolytic, and you have low value feedback resistors).

BTW, love the name we called our daughter Melissa, you probably already know it means 'honey bee'.

Melissa said:

input capacitance < 5pF.

Click to expand...

Does the input capacitance of the BC549 meet the requirements??
I suggest a low input capacitance FET input pair.

The main reason that a cascade (cascode?) pair of differential transistors was used is to avoid Miller multiplication of the transistor's collector to base capacitance. All the other transistors still have the Miller problem which reduces the high frequency response.
The input capacitance of a transistor is less than stray wiring capacitance.

This is still true with cascoded FETs. And they feature very low loading and have smaller capacitances if compared to BJTs.

eng1 said:

This is still true with cascoded FETs. And they feature very low loading and have smaller capacitances if compared to BJTs.

Click to expand...

But JFETs don't have nearly the transconductance of BJTs, so you need higher load resistance to achieve the same gain, so the lower capacitance is of less benefit than one might think.