Piezo Transducer JFET Buffer

Thread Starter

EMFELAB

Joined Feb 11, 2013
10
I need to interface a Piezoelectric transducer sensor which will pick up frequencies up to 1000Hz. It was advised to me to use a JFET source follower circuit. The piezo transducer is simulated here as a AC source in series with a capacitor (10nF), as I understand the Piezo needs a very high input impedance so I haven't connected a RG across the input of the JFET since that would effectively bring down the input impedance of the circuit.

When simulating this circuit, the output gives me 0V and I assume that the biasing is not right; placing a resistor across the gate brings down the signal to 0V DC but the output is still 0V. Now, if I get rid of C3 and R2 and I measure from the source, I get a signal with lower amplitude than the input and with a DC offset. Any suggestions??

 

Ramussons

Joined May 3, 2013
1,032
I need to interface a Piezoelectric transducer sensor which will pick up frequencies up to 1000Hz. It was advised to me to use a JFET source follower circuit. The piezo transducer is simulated here as a AC source in series with a capacitor (10nF), as I understand the Piezo needs a very high input impedance so I haven't connected a RG across the input of the JFET since that would effectively bring down the input impedance of the circuit.

When simulating this circuit, the output gives me 0V and I assume that the biasing is not right; placing a resistor across the gate brings down the signal to 0V DC but the output is still 0V. Now, if I get rid of C3 and R2 and I measure from the source, I get a signal with lower amplitude than the input and with a DC offset. Any suggestions??

We are all blinded without a schematic.

Ramesh
 

MrChips

Joined Oct 2, 2009
23,966
Change your circuit design to a common source circuit.
You will need three resistors, one for the drain, source, and gate.
 

MrChips

Joined Oct 2, 2009
23,966
Note that your PN4392 is an N-channel depletion mode FET.
There are four combinations, P-channel vs N-channel and depletion mode vs enhancement mode.

For depletion mode, you need to bias the gate -ve with respect to the source in order to pinch off the channel. You do this with a resistor from the source to ground. You can add a 10MΩ resistor from the gate to ground to give the gate a reference voltage of 0V.
 

DickCappels

Joined Aug 21, 2008
7,569
If you just want an approximately unity-gain buffer, the circuit you already have should work fine, EXCEPT...you need to put a 1 meg to 10 meg resistor from the gate to ground so that the source voltage and be used to provide negative feedback to establish the FET's operation point.
 
Last edited:

Thread Starter

EMFELAB

Joined Feb 11, 2013
10
Mr Chip. I do not want a common source configuration, a source follower will do as I need a very high impedance. The simulation will not work if I do not bias the gate properly and this means having a gate resistor in parallel.

At the beginning I did not want to do it since it would mean having a resistor parallel to the very high input resistance of the JFET and so lowering it. Therefore, as I understand, the input impedance is simply RG.
 
Top