Insulated-gate field-effect transistors

● Such devices are sometimes called IGFETs (insulated- gate field-effect transistors) or sometimes MOSFETs (metal oxide semiconductor field-effect transistors)
● Digital circuits constructed using these devices are usually described as using MOS technology
● Here we will describe them as MOSFETs

MOSFET

● Construction
– two polarities: n-channel and p-channel

● Operation
– Gate voltage controls the thickness of the channel. – Consider an n-channel device
● making the gate more positive attracts electrons to the gate and makes the channel thicker – reducing the resistance of the channel. The channel is said to be enhanced
● making the gate more negative repels electrons from the gate and makes the channel thinner – increasing the resistance of the channel. The channel is said to be depleted
– The effect of varying the gate voltage



● Devices as described above are termed depletion- enhancement MOSFETs or simply DE MOSFETs
● Some MOSFETs are constructed so that in the absence of any gate voltage there is no channel
– Such devices can be operated in an enhancement mode, but not in a depletion mode (since there is no channel to deplete)
– These are called Enhancement MOSFETs
● Both forms of MOSFET are available as either
n-channel or p-channel devices

● MOSFET circuit symbols

Junction-gate field-effect transistors

● Sometimes known as a JUGFET
● Here we will use another common name – the JFET
● Here the insulated gate of a MOSFET is replaced with a reverse-biased pn junction
● Since the gate junction is always reverse-biased no current flows into the gate and it acts as if it were insulated

JFET

● Construction
– two polarities: n-channel and p-channel


● Operation
– The reverse-biased gate junction produces a depletion layer in the region of the channel
– The gate volt controls the thickness of the depletion layer and hence the thickness of the channel
– Consider an n-channel device
● the gate will always be negative with respect to the source to keep the junction between the gate and the channel reverse- biased
● making the gate more negative increases the thickness of the depletion layer, reducing the width of the channel – increasing the resistance of the channel

– the effect of varying the gate voltage



● JFET circuit symbols

FET characteristics

● While MOSFETs and JFETs operate in different ways, their characteristics are quite similar
● Input characteristics
– in both MOSFETs and JFETs the gate is effectively insulated from the remainder of the device
● Output characteristics
– consider n-channel devices
– usually the drain is more positive than the source
– the drain voltage affects the thickness of the channel




● FET output characteristics



● Transfer characteristics
– similar shape for all forms of FET – but with a different offset
– not a linear response, but over a small region might be considered to approximate a linear response



● Normal operating ranges for FETs


● When operating about its operating point we can describe the transfer characteristic by the change in output that is caused by a certain change in the input
– This corresponds to the slope of the earlier curves
– This quantity has units of current/voltage, which is the reciprocal of resistance (that is conductance)
– Since this quantity describes the transfer characteristics it is called the transconductance, gm



● Small-signal equivalent circuit of a FET
– models the behaviour of the device for small variations of the input about the operating point


● FETs at high frequencies
– at high frequencies more sophisticated models are used

FET amplifiers

Watch the Video  📹

● Simple amplifiers can be formed using any kind of FET
– Figure (a) shows a circuit using a DE MOSFET
– Figure (b) uses an enhancement MOSFET
– Figure (c) uses a JFET
– Figure (d) is a generic circuit that could use any FET
– These are common source amplifiers

Equivalent circuit of a FET amplifier

Watch the Video  📹

● This circuit can represent any of the FET amplifiers above (by choosing an appropriate value of RG)
– This is a small signal-equivalent circuit
– Note that RD goes to ground, since the supply voltage VDD is a virtual earth point for small signals

Small-signal voltage gain

● From the equivalent circuit we can derive the small-signal voltage gain
v0= -gmvgs (rd//RD)
-gmvi (rd//RD)

therefore:

\frac{v_0}{v_i}= -g_m(r_d//R_D)

● Also

r_i\approx R_G       r_0\approx r_d//R_D

● In many cases rd >>RD so rd can often be ignored


● If this is the case

Biasing considerations

● The biasing arrangement determines the operation of the circuit
– This is its quiescent state
● The quiescent output voltage vo(quies) is given by
V o(quies) = VDD -ID(quies)RD
● However, since the FET is not linear, determining the quiescent conditions is not straightforward
● One approach is to use a load line

Choice of operating point

● When selecting an operating point we need to avoid certain forbidden regions in the FET’s characteristics

Device variability

● FETs, like all active devices, suffer from variability



● The effects of device variability on the quiescent conditions of a circuit can be tackled using feedback
– for example, the use of ‘automatic’ bias
– see Examples 18.3 and 18.4 of the course text

A negative feedback amplifier

Watch the Video  📹

● Feedback can be used not only to stabilise the biasing conditions of a circuit, but also its voltage gain

● Analysis of this circuit (see text) shows that


– characteristics set by stable passive components

Use of a decoupling capacitor

● The feedback amplifier has a relatively low gain
● Use of a decoupling capacitor can increase the gain by removing small-signal feedback
– Gain is similar to that of the common source amplifier
– Requires large CS at low frequencies

Source followers

● These are unity-gain amplifiers with a low output resistance

Differential amplifiers

● A simple differential amplifier using JFETs – this is termed a long-tailed pair amplifier

● Input voltage is vi = v1 - v2
● Output voltage is vo = v3 - v4
● Voltage gain is

= \frac{v_o}{v_i} = \frac{v_3 - v_4}{v_1 - v_2}\approx -g_mR_D

● CMRR is - CMRR≈ gmRS

Other FET applications

● FETs as constant current sources

● A long-tailed pair amplifier with a FET current source
– the current source gives the effect of having a very high source resistor and therefore gives a very high CMRR


● A FET as a voltage-controlled resistance
– for small drain-to-source voltages FETs resemble voltage-controlled resistors
– the gate voltage VG is used to control this resistance
– can be used in a potential divider (as shown here) to produce a voltage-controlled attenuator


● A FET as an analogue switch


● A FET as a logical switch


● CMOS circuits
– uses both p- and n-channel devices
– resembles two switches in series
– low output resistance in either state

FET circuit examples

● FET input buffer for an operational amplifier

● An integrator with reset

● A sample and hold gate – basic principle



– gate with input and output buffers



● A FET sample and hold gate

Key points

● FETs are widely used in both analogue and digital circuits

● They have high input resistance and small physical size

● There are two basic forms of FET: MOSFETs and JFETs

● MOSFETs may be divided into DE and Enhancement types

● In each case the gate voltage controls the current from the drain to the source

● The characteristics of the various forms of FET are similar except that they require different bias voltages

● FETs can be used in a range of amplifier configurations

● FETs can be also used to produce other circuit applications

right