Charge Amplifier Question

Thread Starter

Albert1999

Joined May 21, 2019
2
Hi everyone.
I am designing charge amplifier and I'm trying to figure out what is the highest possible amplification i can get, because I'm working with microvolts. Are there any boundaries? The only one i can think of is that GBW of amplifier can provide that amplification. Also, are there any boundaries with choosing feedback resistor? What i found was that at lowest frequency of interest it must be bigger than impedance of feedback capacitor. And the last question is how can I simulate the effect of input resistance of an amplifier on charge amplification? Couldn't find anything else than fact that it's better with high input resistance amps like cmos or jfet.
As you can see I'm having trouble with finding good handbook on this problem. If you have some you can recommend it would be great, I was using Piezoelectric Accelerometers and Vibration Preamplifiers (theory and application handbook) although I'm working with hydrophone. Thank you!
 

shteii01

Joined Feb 19, 2010
4,647
There is voltage amplifier.
There is current amplifier.
There is power amplifier (amplifies voltage and current).

What is charge amplifier?
 

OBW0549

Joined Mar 2, 2015
3,092
Typically, with a charge amplifier for use with piezo accelerometers, you set the charge sensitivity, in volts/picocoulomb, by selecting the feedback capacitor. The feedback resistor is chosen so that the low-frequency rolloff is correct for that value of feedback capacitor.

I've used the LT1793 op amp in several commercial charge amplifier designs; the data sheet shows a diagram for an accelerometer amplifier on the last page.
 

Wolframore

Joined Jan 21, 2019
1,479
Very interesting. Didn’t know what they were called. Found a good source. Part 2 and 3.

http://www-physics.lbl.gov/~spieler/Heidelberg_Notes/

Wow they are quite amazing. Do you have a need to have more gain than available? You can add a stage. The gain depends on the source capacitance. Feedback resistor will affect drain time of feedback capacitor so it would affect frequency response. The real purpose is there to remove DC drift.
 

Thread Starter

Albert1999

Joined May 21, 2019
2
Thanks for the source it seems good, I will look more into it.
I know I can add stage, I'm just trying to figure out are there any other concerns about gain Cs(sensor)/Cf(feedback) or how large can it be. If Cf gets to low whats the consequence...
 

Wolframore

Joined Jan 21, 2019
1,479
I'm also reading more about this interesting circuit... so simple but elusive and very interesting... there is a direct relationship between output voltage and Cfb feedback Cap described as Qin (input charge)/Cfb... in practice the gain is usually 100,000 - 10,000,000. the drift compensation method will affect the freq measured... so your Cfb and Rc will affect this circuit... The Rc is there to remove the drift but has the effect of creating a parallel RC filter for the current transfer R/(1+sRC). small enough to improve gain and large enough not to get overloaded...
About as far as I got for now...
 

OBW0549

Joined Mar 2, 2015
3,092
One Cfb/Rfb combination often used is 1000 pF and 1.0 GΩ. This gives a low-frequency rolloff of 0.16 Hz which is good for most work. Some random notes on charge amplifier design:

Watch out for op amp instability/oscillations caused by the capacitance of the transducer being connected directly to the (-) input of the op amp; it helps to put a small resistor in between the transducer and the op amp input. A couple hundred ohms should be enough.

A JFET input op amp or CMOS op amp is a must for charge amplifiers; BJT-input op amps have too much input bias current, which creates large offset voltages given the high values of Rfb used. So don't go using a 741 or LM358.

Keep in mind that piezoelectric transducers are sensitive to thermal transients. Sudden changes in temperature will produce an output from the transducer that can be quite large. In extreme cases, some kind of thermal isolation may be needed.
 
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