replacing a FET transistor buffer with an OP Amp ?

hi guys i recently saw this gamma detector circuit , i have all the components except the FET which i cannot find , and i was wondering if i can replace the FET buffer with just an lm324 buffer ?

the circuit :

another little question , is the 2N3904 in the common base here just for stabilizing the input voltage and reduces ripple ?

The FET is used for its high input impedance - commensurate with the 10 Megohm load resistor. This is higher than the LM324. You can get FET input op-amps, but you could probably use a CMOS IC (such as a CD4000) to do the job (with a different circuit).

ahmed44:
another little question , is the 2N3904 in the common base here just for stabilizing the input voltage and reduces ripple ?

Looks like it. I am not sure just what it is supposed to be doing. :astonished:

It’s an old trick. It has a filtering effect equivalent to the 100 microfarad capacitor multiplied by the gain of the transistor.

is the 2N3904 in the common base here just for stabilizing the input voltage and reduces ripple

Yes, and probably not necessary for that circuit. A 1K/100uF decoupling filter should be adequate.

The JFET used in that circuit is not critical and you should be able to find substitutes. Check the data sheet here and look for JFETs with similar characteristics, like input capacitance and common source forward transconductance.

Paul__B:
The FET is used for its high input impedance - commensurate with the 10 Megohm load resistor. This is higher than the LM324. You can get FET input op-amps, but you could probably use a CMOS IC (such as a CD4000) to do the job (with a different circuit).
Looks like it. I am not sure just what it is supposed to be doing. :astonished:

the problem that i don't have a FET in hand right now , so i can't get a way with another buffer circuit using the 358 of the 324 other than this "FET" thing .

the transistor circuit was used even tho he indicated the use of 9v battery in the website which i assume is ripple free! , if i used a 9v adapter does it help in some way or i shall remove it ?

The basic circuit design requires a high impedance buffer, either a FET or a FET input op amp.

The circuit is supposed to light an LED and activate a piezo sounder when a gamma ray is detected, and both of those operations will introduce noise into the power supply. So, you must have good decoupling for the photodiode bias supply, or the circuit will just oscillate.

I suspect that the 2n3094 voltage stabilizer is overkill, but I might be wrong.

The inside of an electret capsule is a good source for a free JFET transistor.
Leo..

thanks guys so much for helping i ordered some BF244 and BF245 FETs do they going to cut the job ? , i ordered also a TLC272 op amp after they arrive i will try them .

a little question : is this project suitable as a senior graduation project for a physics major ? or i will need something much more sophisticated .

Ask your professors. They are the ones who decide.

This is a very simple circuit, so they might not be impressed unless you make a significant intellectual contribution in using it.

ahmed44:
a little question : is this project suitable as a senior graduation project for a physics major ? or i will need something much more sophisticated .

Where's the science? I only see engineering.

vaj4088:
It's an old trick. It has a filtering effect equivalent to the 100 microfarad capacitor multiplied by the gain of the transistor.

Called a capacitance multiplier, and its an emitter follower, not common-base. In common base the output is on the collector and the input on the emitter. Here the input is the filtered supply on the base, output is the emitter, so emitter-follower.

aarg:
Where's the science? I only see engineering.

Clearly in the gamma rays being detected by the circuit. The diode BPW34 is a photo detector.