FET amplifiers

FET amplifiers

   FETs are particularly useful as low-noise devices in high input resistance amplifiers.
Before we consider amplifier circuits, the biasing of a FET must be investigated.
In this treatment, an n-channel (depletion) JFET is assumed but the techniques
described are applicable equally to p-channel and to enhancement devices.

Biasing
   Although shunt feedback biasing can be employed, the consequent reduction of
input resistance destroys one of the major assets of FETs in amplifier circuits. To
preserve a very high input resistance the following techniques may be used.
   Voltage bias. In the basic common-source configuration of Fig. 7.8a the source is
connected directly to earth and the gate-source voltage is established by connecting
the gate via a high-valued resistor (RG) to a negative supply voltage (VGG). If the
gate leakage current (IGSS) is zero,
           VGS=VGG

which is the equation of a bias line shown superimposed on the transfer characteristic
limits in Fig. 7.8b. Owing to spread of the characteristic (Vp and IDSS), the quiescent
drain current (IDS) is subject to wide variation from device to device, ranging from
IDS(min) to IDS(max) as shown. At temperatures where the voltage dropped across RG
due to IGSS is significant, the bias line equation is modified to

which introduces a further variability of bias current.


Automatic (or self) bias. As in BJT biasing, in order to aid bias stability, a resistor
R(s) can be introduced into the source lead (Fig. 7.9a). Again the gate is returned
to a voltage supply (VGG) via resistor RG.
The bias line equation is now

and is shown plotted against the transfer characteristic in Fig. 7.9b; its slope is -1/
RS. Several observations can be made. First, considering the case where VGG=0 V
and assuming IGSS=0, it is evident that, due to the finite slope of the bias line
compared with that in the voltage bias circuit, the difference between the maximum
and minimum limits of quiescent IDS is reduced. Second, for the same design value
of drain current, if VGG is taken positive and RS correspondingly increased, the
spread of bias current around its nominal value becomes smaller at the expense of
a higher voltage drop across RS. The positive gate supply voltage can be derived
by a resistive potential divider between the drain supply (VDD) and earth.
If IGSS is significant, the drain bias current is affected as shown by Equation
7.18 but for a given value of RG (which, in the case of a potential divider, is the
parallel combination of the two resistors) the effect is reduced with increasing RS.
This automatic bias circuit is preferred for biasing FETs whether n-channel or
p-channel, depletion or enhancement. For common-source operation, RS must be
capacitively decoupled otherwise the series negative feedback reduces the signal
voltage gain.



Current sources
   Like a BJT, a FET biased in common-source behaves as a current source with a
moderately high output resistance (rds). If gate and source are connected together,
the drain current is IDSS provided that |VDS| is greater than |Vp| or |VTH|. A much
higher output resistance is achieved when an undecoupled source resistor RS is
included; in this case IDS is less than IDSS. Since in a d.c. current source circuit no
a.c. signal is applied to the gate, the gate resistor (RG) may be made a short-circuit.
Prove that the a.c. output resistance of a FET biased with an undecoupled source
resistor RS is

Temperature stability
   JFET transfer characteristics are temperature dependent but exhibit the curious
phenomenon of a certain (IDS, VGS) point being independent of temperature.
Unfortunately this zero-temperature-coefficient bias point occurs rather close to
pinch-off where IDS and gm are usually too small to be of value.



Common-source amplifier
   The circuit of a common-source FET amplifier is shown in Fig. 7.11a. It is left to
capacitors omitted) and to prove that

Also, rin=RG.

Series feedback amplifier
   If series negative feedback is introduced by including an undecoupled source resistor
(RS) as shown in Fig. 7.11b, the voltage gain is reduced to

And, again, rin=RG.
In design, the gate resistor (RG) should be chosen to meet the input resistance
specification, provided that the effect of IGSS and its temperature dependence does
not unduly upset the bias conditions. In order to meet stringent bias stability and
high input resistance specifications it may be necessary to use the bootstrap bias
technique which enhances a low-valued bias resistance (for d.c.stability) into a high
a.c. signal input resistance.

Source follower
   If the output signal is taken from the source terminal, the source-follower circuit
of Fig. 7.11c results. The voltage gain of this circuit is given by

   Since the gm for a FET is usually much lower than that of a BJT, any attempt to
use a high value of RS would appear to prove useful in achieving a voltage gain
close to unity. However, for a constant voltage across RS, as RS is increased so the
drain bias current must decrease and gm also decreases in sympathy. A maximum
gmRS product usually is realized for only moderate values of RS, and the voltage
gain is unlikely to exceed 0.9. The solution to this problem is to replace RS with a
high output resistance current source thus achieving a high effective resistance in
the source with a minimal d.c. voltage drop.
  It can be proved that the output resistance of the source-follower is:

which, again owing to the relatively low gm of a FET, is significantly higher
than the output resistance of the emitter-follower counterpart.
The input resistance of the simple source-follower is equal to RG but bootstrap
bias techniques can raise this value.
Prove the voltage gain expressions for the common-source, series feedback and
source-follower amplifiers given in Equations 7.20, 7.22 and 7.24. Also prove that
the output resistance of a source-follower is as given in Equation 7.25.

Differential amplifier
   Matched FETs can be used in a differential amplifier configuration to achieve a very
high a.c. input resistance (1012 ) and extremely low d.c. input currents (30 pA).
FET and bipolar technologies have been combined in BIFET operational
amplifiers, the FET input stage improving the input performance compared with
all-bipolar circuits. Operational amplifiers using only MOSTs are also available.

Fuente: "Transistor Circuit Techniques" G. J. Ritchie Third Edicion
Asignatura: EES
Nombre: María José Nieto Cárdenas

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