Bias Circuit

11.1.1 Voltage Gain of the Noninverting Resistor Feedback Amplifier
In the circuit diagram of Fig. 11.3, an input signal voltage Vs is attached to resistor Rx. By
definition, the output terminal voltage is positive for a plus input. Hence, this is the
noninverting amplifier. Here we obtain the relationship between the amplifier gain, avo = Vo/Vs,
and the open-loop gain (opamp gain), avo = Vo/V , and the circuit resistors.
Figure 11.3. Non-inverting or voltage amplifier (series-shunt). The
input resistance is essentially infinite.
With a signal applied to the plus input terminal, the responding output voltage is fed back to
the resistor Ry. The voltage across Ry, Vf, and Vo (signal) are related by
Equation 11.1
This is just the voltage-divider relation, which applies in this case, as negligible current flows
into the input terminals of the opamp. Variable Vf is used in lieu of VRy to distinguish it from
the dc value of the voltage across Ry. This use is also consistent with the fact that Vf is
technically a signal feedback voltage.
The input signal voltage Vs adds up to
Equation 11.2
where (11.1) is used to eliminate Vf. (The voltage drop across Rx is essentially zero.) This leads
directly to the relation for amplifier gain, which is
Equation 11.3
with (ideal noninverting gain).