A guide to using FETs for voltage controlled circuits, Part 1

-October 13, 2017

Editor's note: I am so happy that there are still companies around that create precision, discrete transistors in our industry; Linear Integrated Systems is one of the best I have encountered. There are so many applications for the need to design circuitry using quality discrete components instead of integrated circuitry. This multi-part article will show the many advantages of doing these types of designs.
Steve Taranovich

Linear Integrated Systems manufactures a variety of FETs (field effect transistors). In particular they have a variety of matched dual products. There are advantages in having matched devices. For example, if you are building a two-channel stereo audio product, having two or four devices in the same package allows for the two audio channels to be more closely matched.

This paper will explore using FETs in voltage controlled circuits.

Several approaches will be shown:

  1. Using FETs as voltage controlled resistors.
  2. Using FETs as voltage controlled amplifiers and active mixers.
  3. Using FETs as voltage controlled phase shifters for processing music.
  4. Using FETs as voltage controlled band pass filters.

[See more circuitry using discrete FETS here: Building a JFET voltage-tuned Wien bridge oscillator.]

We will also explore ways to reduce nonlinearities or distortions and automatically bias the FETs.

FET voltage controlled resistors

Figure 1 shows a typical current-voltage relationship of an N Channel FET.

Figure 1
A typical N-Channel FET I/V curve for different gate-to-source voltages, VGS1, VGS2, and VGS3.

An FET has basically two regions:

The saturation region, which includes each of the horizontally flat portions where the FET acts as a voltage-controlled current source and the other region that includes the sloped “curved portions” is the triode or ohmic region where the FET can operate as a voltage-controlled resistor. If we look carefully we will notice that the triode region in Figure 1 is shown for drain to source voltages (VDS) that are non-negative.

Note: The triode or ohmic region in an FET is sometimes known as the linear region. The FET operating as a voltage-controlled resistor (VCR) works in this region. Preferably, there is no DC voltage across the FET’s drain and source terminals in the VCR mode.

If we extend the VDS voltage range to include slightly negative voltages for a particular gate-to-source voltage, we see that the there is still a resistive effect (Figure 2).

Figure 2 FET’s triode region extended to a negative VDS voltage, - VDS1, that still shows a resistance effect.

The slope is defined as:

Slope = ΔID/ΔVDS = gds = conductance between the drain and source.

And the resistance across the drain and source is the reciprocal of the conductance,

Rds = 1 / gds = ΔVDS/ΔID

As we look at the two slopes that denote gds, S1 and S2, we will see that they are approximately the same. But if we look very closely, they actually are slightly different with the S2’s slope being steeper than slope S1. A steeper slope yields a higher conductance, which results in a lower resistance. For example, the resistance around the high sloped region around S2 or – VDS1 is lower than the resistance around S1 or + VDS1. The gradual change in resistance from +VDS1 to – VDS1 results in distortion. Fortunately, the distortion can be kept small.

For instance, with small AC signals (e.g., < 500 mV peak to peak) across the drain and source, the harmonic distortion can be “reasonably” low. As an example, if the AC signal voltage across the drain and source is between – 250 mV and +250 mV, then the harmonic distortion will be “small”, typically < 3%.

At this point, one may ask if are there specific FETs made just for voltage controlled resistor applications? The answer is yes (e.g., VCR11), but it turns out that virtually any other FET (e.g., JFET and MOSFET) can be used as a voltage controlled resistor.

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