Op-Amps

What does an Op-Amp do?

An Operational Amplifier essentially scales an input signal (V_in) to an output signal (V_out) by a factor of A.

V_in = | V_- - V₊ |

Key features of an Op - Amp

• Active device, so has two power supplies
  {+V_CC and -V_EE}

• Output signal (V_out) can only range between +V_CC and -V_EE.

  • Usually ±15V

Ideal Op-Amp Characteristics

• R_in = Infinite Ohms

  • Leads to I_in = 0A

• R_out = 0 Ohms

• Open loop Gain (A) is infinite

• A is independent of frequency

• Ideal Op-Amp is also unaffected by temperature

• Infinite slew rate

  • Change in V_in leads to instant change in V_out

THIS IS ONLY FOR IDEAL OP AMPS

Virtual Short

For an ideal Op-Amp:

• A → Infinity

• V_out → Finite Value

• V₊ = V_- {Signifies a virtual short}

Types of Amplifiers

1. Inverting Op-Amp

2. Summing Op-Amp

3. Transresistance Op-Amp

4. Integrator

5. Differentiator

6. Voltage Follower

7. Non-Inverting Op-Amp

8. Transconductance Op-Amp

Inverting Op-Amp

Inverts the polarity of an input voltage

Summing Op-Amp

Sums multiple input voltages together

Transresistance Op-Amp

A Current-to-Voltage converter

Integrator

Output voltage is the integral of input voltage

• -1/RC is a constant

Also acts like a lowpass filter

Problem/Solutions with an Integrator in real life

Problem:

1. At low frequencies, loop gain goes to infinity

Solutions:

1. Use Large values of R and C

2. Zero output regularly, but placing a switch across the capacitor

3. Add a large resistor across the capacitor

   • {This is now just an inverting amplifier at low frequencies, not an integrator}

Differentiator

Output voltage is the derivative of the Input Voltage

• -RC is a constant

Also acts as a highpass filter

Problem/Solution with a Differentiator in real life

Problem:

• High gain at high frequencies, and can lead to unstable output

Solution:

• Place a resistor in series with input capacitor
  {Acts as an inverting Op-Amp at high frequencies, not a differentiator}

Voltage Follower

Ensures Output Voltage = Input Voltage

Acts as a buffer

Non-Inverting Op-Amp

Output voltage is a positive multiple (A) of the Input Voltage

A is the gain

Transconductance Op-Amp

A Voltage-to-current converter

Input Bias Current

In real life, the terminals on the Op-Amps have a current flowing through them

This can be compensated by connecting a specific resistor (R_B) where the input bias current goes to 0A.