Question

g Design single input op-amp circuits with with closed loop gains of −10, +10, +1 and −1. Use 10kΩ for the feedback resistor Rf in your designs. What type of feedback is used in the ±10 gain amplifiers? Explain with appropriate equations the effect of feedback on input and output resistances of the amplifier circuits.

Answer 1

Explanation:

For Gain of −10

Inverting op-amp gain is given by

$Gain=V_{out} /V_{in} = -R_{f} /R_{in}$

The negative sign represents output signal being phase shifted by 180°

$-10=-10/R_{in}$

$R_{in} =-10k/-10$

$R_{in} =1k$Ω  

For Gain of −1

$Gain= -R_{f} /R_{in}$

$-1=-10k/R_{in}$

$R_{in} =-10k/-1$

$R_{in} =10k$Ω

For Gain of +10

Non-inverting op-amp gain is given by

$Gain=V_{out} /V_{in} =1 + R_{f}/R_{in}$

$10=1+(10k/R_{in})$

$10-1=10k/R_{in}$

$9=10k/R_{in}$

$R_{in} =10k/9$

$R_{in} = 1.11k$Ω

For Gain of +1

To get a Gain of +1, make $R_{f}=0$  which results in $R_{in} =$ ∞

The output is directly connected to the input which makes $V_{out}$ equal to $V_{in}$Therefore, giving a gain of exactly 1.

$V_{out} =G*V_{in}$

$G=1$

This kind of design configuration is also known as Voltage Follower.

Negative feedback is being used to design these op-amp amplifiers.

$\frac{V_{out}}{V_{in}} =\frac{G}{1+\beta G}$

Negative feedback increases the stability at the cost of reduced gain.

The gain is reduced by a factor of $1+\beta G$  

Negative feedback reduces noise and distortion and improves frequency response of the amplifier.

An ideal op-amp should have infinite input resistance and zero output resistance. Negative feedback increases the input impedance and reduces the output impedance.