op-amp applications wk2

Op-amp Applications

differential amplifier high-gain amplifier

voltage

level

shifter

output

stage

current mirror

current mirror current mirror

Differential Amplifier

The resultant output voltage is given by

Example

1) Design a differential voltage amplifier with va = 3 V, vb = 5 V, Ra = R1 = 12 kΩ

and RF = RX = 24 kΩ

2) Design a differential voltage amplifier with va = 3 V, vb = 5 V and let all

resistance have the same values of 20 kΩ

Voltage Follower

1. The output voltage equals the input voltage: vO = vS. This circuit is commonly

referred to as a voltage follower because its output voltage follows the input

voltage.

2. It has the inherent characteristics of a high input impedance (or resistance, typically

1010Ω) and a low output impedance (or resistance, typically 50 mΩ)

Integrator

The output voltage in the time domain becomes

Practical Inverting Integrator

In practice, as a result of its imperfections (such as

drift or input offset current) an op-amp produces an

output voltage even when the input signal is zero

and the capacitor will be charged by the small by

finite current through it. The capacitor prevents any

DC signal feeding back from the output terminal to

the input. As a result, the capacitor will charged

continuously and the output voltage will build up

until op-amp saturates.

Inverting Integrator

1. A resistor with a large value of RF is normally connected in parallel with the

capacitor of capacitance CF,.

2. RF provides the DC feedback and overcomes this saturation problem.

3. Time constant τF ( = RFCF) must be larger than the period Ts (= 1⁄fs) of the input

signal.

4. A ratio of 10 to 1 is generally adequate; that is, τF = 10Ts.

The output voltage in the time domain

becomes

Example

1. Design an integrator with frequency of the input is

fs = 500 Hz. The voltage gain should be unity at

a frequency of f1 = 1590 Hz. That is, the unity-

gain bandwidth is fbw = 1590 Hz. Let CF = 0.1 µF

2. The integrator in part (1) has Vcc = Vee = -12 V

and the maximum voltage swing = ±10 V. The

initial capacitor voltage is Vco = 0. Draw the

waveform of the output voltage for the input

voltage shown at the white boards.

Noninverting Integrator

The output voltage is given by

Differentiator

1. The value of Rx should be made equal to RF.

2. A differentiator circuit is useful in producing sharp trigger pulses to drive other

circuits.

3. A differentiator circuit behaves like a high-pass network.

4. The output voltage in the time domain is

Practical Inverting Differentiator

1. A modified circuit that is often utilized as a differentiator, in which a small

resistance R1 (< RF) is connected in series with C1 to limit the gain at high

frequencies.

2. This arrangement limits the high-frequency range.

Example Differentiator

The differentiator has R1 = 2 kΩ, RF = 10 kΩ and

C1 = 0.01 µF. Determine (a) the differentiator time

constant, (b) the gain limiting frequency and (c) the

maximum closed loop voltage gain Af(max)