FET Matching

Since the output of the gain-reduction amp circuit feeds two FETS, one for metering and one for signal attenuation, it's a good idea to match these devices so that the GR meter is more easily calibrated to show what is happening to the signal.

Fortunately this is fairly easy to do. Using the simple test jig circuit shown here we can vary Vgs and measure the corresponding Vds value. These can be put into a simple spreadsheet and the resulting characteristic curves plotted.

You will need a power supply capable of providing +/-10V or thereabouts (two 9V batteries will do it) and a DVM to measure the input and output voltages. Make sure you mark each device so that you can identify them later.

2N5457 FET data sheet (pdf) FET matching spreadsheet (excel) 

 

FET match testing circuit

 

 

 

 

 

 

 

 

 

 

 

 

 

Each of 5 devices were tested at Vgs values of:

  • -3.00V
  • -2.00V
  • -1.00V
  • -0.75V
  • -0.50V
  • -0.25V
  • 0.00V

Calculation of Id (mA):

Id = (V - Vds)/R

Where:

V is the supply voltage

Vds is what is measured between the drain and source of the FET.

R is the drain resistor in k.

 

A simple chart was plotted for the 5 devices tested.

 

FET curves 1

 

 

 

 

 

 

 

 

 

 

 

 

 

Luckily there are a couple of pairs that are fairly closely matched here.

Aother ten devices were also tested for comparison. This graph shows quite a spread of devices, however the slope of the linear region of conductance is consistent across all devices.

FET transfer curve