Page 99 - Mechanical Engineers Reference Book

2/40 Electrical and electronics principles
Kirchhoff's first law, it is apparent that il = -i2. Thus VI/
R1 = -V,/Rz, and the gain can be written as
(2.108)

Provided the open-circuit gain of the amplifier is very high,
the overall gain with this negative feedback system is given by
the ratio of the two external resistors and is independent of the
open-circuit gain.

2.3.14.2 Unity gain amplifier
Figure 2.80 depicts a unity gain amplifier in which no external
resistors are wired into the circuit. The unity gain amplifier is
also known as a voltage follower or as a buffer amplifier. This
type of amplifier circuit is often used in instrumentation
systems where the internal resistance of a voltage-generating
transducer and that of the voltage-recording instrument are so
poorly matched that the transducer voltage is seriously atte-
nuated. This situation arises when the transducer internal Figure 2.81 Non-inverting amplifier
resistance is large in comparison to that of the recording
instrument. Since the buffer amplifier has a large input
impedance and a low output impedance it can be interfaced Hence
between the transducer and the recording instrument to vo R2 + Rl
provide optimum impedance matching. This gives a low -
source impedance and high destination impedance between Vi RI
both the transducer and amplifier and also between the Since E is a virtual earth, then Vi = VI and
amplifier and the instrument.
V, R2 + Ri
Summing the voltages round the amplifier in Figure 2.80 --___ (2.110)
-
gives v1 R1
v, + v, = vn If, in addition, R2 9 R1,
Since the internal impedance of the amplifier is very large then - -5
Vn
-
V, is effectively zero and the gain is (2.111)
v1 R1
vo/vl = 1 (2.109)
2.3.14.4 Summing amplifier
The summing amplifier is shown in Figure 2.82. As point E is a
2.3.14.3 Non-inverting amplifier virtual earth, then
Figure 2.81 shows the operational amplifier connected up for a -id = il + iz + i3
non-inverting output. Assuming that the currents through
resistors R1 and R2 are equal and that point E is a virtual earth, Therefore
vi v, - vi VO
-
Rl R2 R4

Figure 2.80 Unity gain amplifier Figure 2.82 Summing amplifier