Bias Circuit

Equation 11.4
AvNI is the limiting form of the noninverting amplifier gain for avo . This is consistent with
the fact that in the limit, V 0 and Vs = VRy. Thus, the output and input voltages are simply
related by the voltage-divider relation. The result, (11.3), indicates that for avo>>AvNI, the
voltage gain can simply be expressed in terms of the resistors and therefore is very
predictable. If we make, for example, Rf = 10Ry, AvNI = 11, and (11.3) gives Av = 10.998, with
avo = 40,000. The value for the opamp gain is typical for our project opamp. Note that due to
the high resistance at the opamp input terminals, Rx has no influence on the gain.
The noninverting amplifier has a very high input resistance and a low output resistance, as
discussed in Unit 11.4. The amplifier technically falls into the category of a series – shunt
feedback configuration or a voltage amplifier.
11.1.2 Voltage Gain of the Inverting Resistor Feedback Amplifier
To obtain the inverting amplifier, the signal is applied to the negative or inverting terminal as
shown in Fig. 11.4. A positive signal results in a negative output voltage. The gain expression
can be obtained with the loop equation from output to input:
Equation 11.5
Figure 11.4. Inverting or transresistance amplifier (shunt – shunt).
The input resistance is equal to the signal-source resistance, Ry.
and the loop equation at the input (voltage drop across Rx is zero)
Equation 11.6
Eliminating Is between (11.5) and (11.6) gives
Equation 11.7