A chance discovery at the Linear Technology web site has led me to a simple method of turning a current feedback amplifier into an inverting integrator, a feat not normally possible by treating the CFB amplifier as a conventional opamp. Using this method, I have designed and simulated some simple discrete-based charge sensitive amplifiers for use with PMTs and possibly other detectors. One circuit uses the CFB amp circuit I posted here a while back, the other uses a simple variant of the Hamamatsu 3-transistor preamp, which is about the simplest example of a current feedback amplifier you'll find anywhere.
You might ask, what is a charge senstive amplifier and why is it useful? A charge sensitive amplifier is a current-input integrator that takes a current signal from a transducer and puts out a voltage proportional to the total charge of the current input signal. The input of the amplifier is a virtual ground, greatly reducing the effects of cable capacitance on the incoming signal . The integrator function helps to filter out high frequency noise, and with the appopriate reset technique, also serves to stretch the input pulse. making life easier for subsequent signal processors. The pulse stretching function makes the CSA attractive for use with plastic scintillators, which put out a light pulse with a ~3-5 nsec decay time. Though NaI scintillators and ZnS scintillators (such as the BC-720) put out a much longer light pulse (~200 nsec), they can benifit from a 2-5X pulse stretch to allow use of a relatively slow discriminator comparator.
When I get a little extra time (ha!) I'll compile the two circuits with simulation results and post them in a PDF file just like I did the last bunch. I don't think I'll actually be building them soon, as my current approach to PMT detector circuitry works well enough to put in the final product. This approach uses an extremely fast discrete voltage feedback amplifier configured as a charge sensitive amplifier followed by a 3-transistor voltage feedback voltage amplifier. I will post the circuit as soon as I can compile some results. I spent a couple of days hunting down and killing a 300MHz oscillation (it's a fast amp, right?), and now can proceed to actual pulse testing.
Simple Charge Sensitive Amplifiers
Re: Simple Charge Sensitive Amplifiers
When Richard?