Operational Amplifier (Op Amp)

Circuit Symbol

𝑉0 𝑉0
𝐴𝑂𝐿 = =
𝑉2 − 𝑉1 𝑉𝑖𝑑
𝐴𝑂𝐿 = Open Loop Gain

Features:
● Can amplify both AC and DC signal
● Both +ve/-ve Signal can be amplified
Pin Layout

ON NC

ON

Pin Description

Pin 1: Offset Null (ON)

Pin 2: Inverting Input Terminal
Pin 3: Non-Inverting Input Terminal
Pin 4: Negative Supply Voltage
Pin 5: Offset Null (ON)
Pin 6: Output Pin
Pin 7: Positive Supply Voltage
Pin 1: Not Connected (NC)
Characteristics of Ideal & Practical Op Amp :

Ideal Practical

Open Loop Gain, 𝐴𝑂𝐿 ∞ ∼ 105 − 106 𝑉/𝑉

Differential Input Resistance, 𝑅𝑖𝑑 ∼MΩ

∞
Output Resistance, 𝑅𝑂 0 ᐸ100Ω

Bandwidth, BW ∼MHz
∞

Equivalent Circuit of a Practical Op Amp

Equivalent Circuit of an Ideal Op Amp

Application of Op Amp in
Open Loop Connection ⇒ 1. Switch
2. Comparator
3.Detector

Close Loop Connection ⇒ 1. Amplifier

2. Adder
3. Differentiator
4. Integrator
5. Filter
6. Oscillator

Ideal Op Amp
When analyzing, we assume an Op Amp as an ideal Op Amp.
Output Voltage, 𝑉0 = 𝐴𝑂𝐿 (𝑣2 − 𝑣1 )
= 𝑉𝑠𝑎𝑡 + , 𝑤ℎ𝑒𝑛 𝑣2 > 𝑣1 𝑎𝑛𝑑 𝑣𝑖𝑑 = 𝑣2 − 𝑣1 is +ve
= 𝑉𝑠𝑎𝑡 − , when 𝑣2 < 𝑣1 𝑎𝑛𝑑 𝑣𝑖𝑑 = 𝑣2 − 𝑣1 is -ve
𝑉𝑠𝑎𝑡 + = 𝑉 + − 𝛥𝑉, Positive Saturation Voltage
𝑉𝑠𝑎𝑡 − = 𝑉 − + 𝛥𝑉, Negative Saturation Voltage
𝛥𝑉 − 𝐼𝑛𝑡𝑒𝑟𝑛𝑎𝑙 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝, 𝑢𝑠𝑢𝑎𝑙𝑙𝑦 𝑎𝑏𝑜𝑢𝑡 1 − 2 𝑣𝑜𝑙𝑡

If 𝑉 + = 12 V, 𝑉 − = -12 V, and 𝛥𝑉 = 2 V
𝑉𝑠𝑎𝑡 + = 𝑉 + − 𝛥𝑉 = 12 - 2 = 10V
𝑉𝑠𝑎𝑡 − = 𝑉 − + 𝛥𝑉 = -12 + 2 = -10V
By adding a DC voltage source in series with 𝑣1 , we can change the length of
time output voltage will remain high, which is also called the ON time

By making the DC voltage at 𝑣1 variable, we can control the length of time output
voltage will remain high. Increasing Vr will decrease the pulse width, whereas
decreasing Vr will increase the pulse width. This process is also known as the
pulse width modulation (PWM) as we are basically varying the width of the
pulses. Thus, through PWM, we are able to vary the ON/OFF time.
The PWM output can be used to control the ON/OFF time of a switch, thus the
flow of power. Thus this circuit can be used to control the flow of power to any
electrical appliances, such as, heater, motor, bulb or any other equipment.

When this circuit is used in switching operation, in most cases, the negative
output voltage may not be necessary, only a low positive might be enough to turn
OFF a switch. In such cases, the negative supply voltage to the op amp can be
eliminated by reducing it to zero. Under this situation, the output of the op amp
will look like as a series of positive pulses as shown in the figure below. The
width of the pulse can be varied by making Vr variable.

If we set 𝑉 − 𝑡𝑜 0, the output will always be positive.

𝑉𝑠𝑎𝑡 + = 𝑉 + − 𝛥𝑉 = 𝑉 + − 𝛥𝑉
𝑉𝑠𝑎𝑡 − = 𝑉 − + 𝛥𝑉 = ∆𝑉 [ where 𝑉 − = 0]
As, 𝑉 − = 0, we call it a single Power Supply connection.
 Vo

Above circuit shows how the power flow to a dc load (in this case a dc bulb) can
be controlled using transistor as a switch. When the output of the op amp is high,
transistor will be ON, and current will flow into the load. When the output goes low,
transistor will turn off, current flow will stop, and the bulb will also turn off.

To limit the current flow into the base of the transistor, a base resistor R B is
connected. Switching operation of the transistor is achieved by operating the
transistor in saturation and cutoff. Saturation mode operation of the transistor is
ensured by forcing a base current IB which is greater than IC/β.

For example, if the full load current of the bulb is 1A and β = 100, then,
𝐼𝐶 1𝐴
𝐼𝐵 == = 0.01𝐴 = 10 𝑚𝐴
𝛽 100
𝑉𝑜 − 𝑉𝐵𝐸
𝑅𝐵 <
𝐼𝐵
If we take a value of RB which is less than the calculated value using the above
expression, this will ensure current RB greater than 10mA will flow into the base
and transistor will operate in saturation.
In order to accurately design the switching circuit, it is necessary to fix the output
voltage Vo. For this purpose, a Zener diode is connected at the output which will
maintain a fix voltage as dictated by the Zener voltage. If we connect a 5-V Zener
diode, voltage will be fixed at 5V. Then the value required for RB will be given as,
𝑉𝑍 − 𝑉𝐵𝐸 5 − 0.7
𝑅𝐵 < = = 0.43 𝑘𝛺 = 430𝛺
𝐼𝐵 10
From the above equation, it is clear that we need to choose a value for RB which is
less than 430Ω.
In the circuit, transistor used has to be a power transistor, as it needs to handle
large power, in the range of watts, and large current, in the range of ampere.
Transistor available in the labs can handle current in the range of mA and power
in the range of mW.
To control the power flow into an AC load, an electro-mechanical relay switch can
be used, since through a transistor current can flow only in one direction. In other
words, transistors can not handle the AC power.

The diagram below shows the connection diagram of the relay switch. The coil of
the relay is connected in the path of the collector current flow of the transistor.

When the output voltage Vo of the op amp is high, transistor turns ON, collector
current flows, energizing the relay coil, relay switch closes, and load is connected
to the AC supply. Since this is a mechanical switch, current can flow in both
directions. When the output voltage Vo of the op amp goes low, transistor turns
OFF, collector current stops, de-energizing the relay coil, relay switch turns open,
disconnecting the load from the supply.
Power Transistor :
They have a material core in the lab plastic core.
Relay is a mechanical SWITCH.
When signal gets high → Current will flow → Switch will be closed.
When signal gets low → Current will not flow → Switch will be opened.