Dual gate MOSFET

- the dual gate MOSFET, Metal Oxide Semiconductor field effect transistor offers additional capability and flexibility of operation over the standard MOSFET: with two gates it has two control terminals.

One form of MOSFET that is particularly popular in many RF applications is the dual gate MOSFET.

The dual gate MOSFET is used in many RF and other applications where two control gates are required in series.

The dual gate MOSFET is essentially a form of MOSFET where two gates are fabricated along the length of the channel - one after the other. In this way, both gates affect the level of current flowing between the source and drain.

In effect, the dual gate MOSFET operation can be considered the same as two MOSFET devices in series. Both gates affect the overall MOSFET operation and hence the output.

Circuit symbol for a dual gate MOSFET - n-channel.
Dual gate MOSFET circuit symbol

The dual gate MOSFET can be used in a number of applications including RF mixers /multipliers, RF amplifiers, amplifiers with gain control and the like.

Dual gate MOSFET structure

The dual gate MOSFET has what may be referred to as a tetrode construction where the two grids control the current through the channel.

The different gates control different sections of the channel which are in series with each other.

The structure of a typical dual gate MOSFET showing the different regions within the device.
Dual gate MOSFET structure

Dual gate MOSFET amplifier

Dual gate MOSFETs are able to operate with improved performance as amplifiers over single gated FETs. The dual gate MOSFET enables a cascode two stage amplifier to be constructed using a single device.

The cascade amplifier helps overcome the Miller effect where capacitance is present between the input and output stages. Although the Miller effect can relate to any impedance between the input and output, normally the most critical is capacitance. This capacitance can lead to an increase in the level of input capacitance experienced and in high frequency (e.g. VHF & UHF) amplifiers it can also lead to instability.

The effect is overcome by using a cascade amplifier using a single dual gate FET. In this configuration, biasing the drain-side gate at constant potential reduces the gain loss caused by Miller effect. The effects of capacitive coupling between the input and output are virtually eliminated.

The method of implementation of the dual gate MOSFET amplifier can be seen in the diagram below.

In this circuit the lower or input FET section is in a self-biased, common-source configuration. The upper or output FET section is configured in a in a voltage-divider biased, common-gate configuration.

Circuit for a dual gate MOSFET amplifier often used in RF applications.
Dual gate MOSFET amplifier

Effectively a cascade amplifier is a two-stage amplifier formed from a trans-conductance amplifier which is followed by a current buffer. This provides a high level of input-output isolation, high input impedance, high output impedance, higher gain or higher bandwidth when compared to a single stage amplifier.

A cascode amplifier using a dual gate MOSFET is commonly used in radio receiver front ends. In these applications, the dual-gate MOSFET is operated as a common source amplifier with the primary gate, i.e. gate 1, G1 connected to the input and the second gate, G2 grounded.

Dual gate MOSFET mixer

The dual gate MOSFET is able to provide a basis for an RF mixer. The dual gate MOSFET operation enables both the local oscillator and RF signal inputs to be accommodated. As shown in the circuit below, the RF signal is normally applied to gate 1 and the local oscillator to gate 2.

The operation of this dual gate MOSFET circuit is relatively easy to understand. The RF signal appears at gate 1 and controls the channel current in the normal way. However the much higher level local oscillator signal is applied to gate 2 and superimposes its effect on the channel current.

Circuit for a dual gate MOSFET mixer for use with RF applications.
Dual gate MOSFET mixer

This dual gate MOSFET mixer is widely used in a number of RF applications, particularly for VHF and UHF applications where they operate well for the limited number of components used.

The dual gate MOSFET is used in many RF applications, both as a mixer and as an RF amplifier. In these and other applications the use of two gates to a single device makes it particularly useful. Although not as widely used as its single gate relative, the dual gate MOSFET is able to provide improved performance in several niche areas.

By Ian Poole

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