Op Amps

Operational Amplifier Definitions

An operational amplifier (op-amp) is a high-gain differential amplifier that amplifies the difference between two input voltages.

Properties of an ideal op-amp:

1. Infinite input impedance
   → No current flows into either input terminal
   → I_in = 0

2. Infinite open-loop gain
   → The output voltage is extremely large for even a tiny difference between inputs
   → V_out = A(V₊ - V_-) where A is infinite

3. Zero output impedance
   → The output voltage is not affected by the load connected
   → Maximum power transfer without voltage drop

Real vs Ideal Operational Amplifier

1. Input impedance

   • Ideal: infinite (takes nothing)

   • Real: very high (but finite)

2. Gain

   • Ideal: infinite (boosts forever)

   • Real: very large, but limited (e.g. 10⁵ - 10⁶

3. Output impedance

   • Ideal: zero (perfect output)

   • Real: small, but not zero

Op-Amp as a Comparator: Operation

If V+ > V- —> Vout = +Vmax (maximum positive voltage, positive saturation)

If V+ < V- —> Vout = -Vmax (maximum negative voltage, negative saturation)

Op-Amp as a Comparator: Applications

Sine → Square wave

• A sine wave goes up and down smoothly

• The op-amp says:

  • Above 0 → HIGH

  • Below 0 → LOW

👉 Result: a square wave (sharp jumps)

Example 2: Temperature alarm

1. Set a reference temperature

2. If temperature:

   • goes above → alarm ON

   • stays below → alarm OFF

👉 Like a “too hot!” warning system

Inverting Op Amp

Non Inverting Op Amp

Closed loop voltage gain (inverting circuit)

= -R_f / R_in

Open loop gain (non inverting circuit)

R_f / R_{in + 1}