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JFET Design Spotlight: Another Variation of a High Z Preamp for Low Level Piezo Sensing 

4 hours ago
2 min read

This week’s Design Spotlight is another great example of how JFETs and precision discrete components can come together to solve demanding analog design challenges.

 

“Though there was another Piezo sensor earlier, this one has a bit more gain and a design bandwidth of 1.3 MHz. This circuit uses, as you’d expect, a low noise LSK389 differential JFET pair cascoded by Q1 & Q2 into a Widlar style current mirror. The LS310 transistor pair maintains constant voltage, low impedance terminations for the differential FETs where gate to drain potential is constant with respect to the signal input. Q1 and Q2 also eliminate the miller effect improving bandwidth in cascode.  An LT1055 OpAmp provides little parasitic high frequency load as a transimpedance amp for the mirror output. A Linear Systems J502 current source diode serves to bias gate and base voltages of the lower two sections through Zener diodes. Another J502 diode supplies a high Z source current for the differential front end. Degeneration resistors are used in the LSK389 sources to linearize input voltage range 10% higher than with a direct source connection. The J500 series fixed current source diodes often find their way into a wide range of circuits from floating voltage resistive drops to most High Z source requirements. Fig 1 shows a -3db response from 100KHz to 1.3MHz by varying Cc from 33pF to 3.3pF. Overall salient design points are : High Z Differential JFET input, Cascoded low Z FET drain isolation, Widlar style current mirror, Transimpedance translator of current to voltage output ……. Not much to it….. But very effective.

 

As Usual ……. Kirkwood Rough”


 



Designing your own high-performance analog front end?

Linear Systems offers the LSK389 matched dual JFET, LS310 matched transistor pair, J500 Series Current Regulating Diodes, and a broad range of precision discrete semiconductors for demanding analog applications.

 

Interested in FREE engineering samples? Contact us to discuss your application and the devices you'd like to evaluate.


 

 

 
 
 

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