Understanding op-amps -2

Op-amp fundamentals

You are studying operational amplifiers. This is the second article that explains op-amp fundamentals.

Prerequisites

  1. Remember the contents of the earlier article. You may refer to the pdf file which you downloaded.
  2. You need to have the practical kit ( Multimeter,Wires,Op-amp LM324, Breadboard, standard value-resistors ) ready so that you can verify the theory easily.

Op-amp applications

Almost all transducers/sensors produce weak output signals. They must be amplified before they are processed further. Op-amp is the only device which can do this thing with great performance.

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Ideal Op-amp

An ideal op-amp should satisfy following conditions:

Infinite Bandwidth

Bandwidth means range of input signal frequencies over which the gain of the amplifier is constant. Op-amp is expected to work with the same (infinite) gain for all frequencies. (Practically this is not possible due to the inter-junction capacitance of transistors used to construct op-amps. Well, this is an ideal op-amp isn’t it ?)

Zero Output impendance

Output impedance decides the amount of current which can be delivered to the load by the op-amp. Lesser is the output impedance, more current it can deliver in to the load. For an ideal op-amp, this is zero. Which means the ideal op-amp can deliver infinite current into the load.

Infinite Gain

An ideal op-amp has infinite gain. Theoretically, a very small input signal can be made very large, using op-amp. Practically, an op-amp has very large gain (in thousands). In practice, input signals are in micro-volts or millivolts. An op-amp makes the output large enough to be compatible with the load on the op-amp.

Infinite Input impedance

Input impedance of a circuit decides how much current it draws from its input signal source. More is the input impedance, lesser current it takes. An ideal op-amp takes no current from the input signal source because it input impedance is infinite. A practical op-amp may draw current in micro-amperes from the input signal source.

Infinite CMRR

CMRR stands for Common Mode Rejection Ratio. This ratio decides how much immune the op-amp is against noise. In theory, CMRR is defined as:

CMRR=\frac{Differential\  mode\  gain}{Common\  mode\  gain}

In simple words,

CMRR=\frac{Gain\ offered\  to\  signal}{gain\  offered\  to\  noise}

In the above formula, if gain offered for noise is less (which is desired), CMRR will be large. An ideal op-amp rejects noise (offers zero gain to noise) and offers infinite gain to the signal. This makes the CMRR infinite. Practical op-amp has very high CMRR.

The above discussion about an ideal op-amp is available in pdf format. You can download it here.

BOGIC

All the above ideal op-amp parameters can be remembered with a funny mnemonic: BOGIC

  • B stands for bandwidth
  • O for output impedance
  • G for gain
  • I for input impedance and..
  • C for CMRR

All the above, except Output impedance, are infinite. The letter O reminds us of zero which is the output impedance.

Op-amp construction simplified

Op-amp is built in three stages:

  1. Differential input stage: Two transistor amplifiers form this stage. One is inverting amplifier and the other non-inverting amplifier. Difference between two inputs is amplified further. The input signal is applied to the op-amp such that the difference between signal voltage and ground is amplified. Noise incident on both inputs is rejected (common mode gain is zero) because it is common to both inputs.
  2. High gain stage: Multiple cascaded amplifiers make the gain very high. This satisfies the infinite gain criterion.
  3. High currrent output stage: Using emitter followers etc, the current delivering ability of the output stage is increased.

The comparator

Now you are going to verify an important configuration of an op-amp. It is the comparator.

Comparator using op-amp LM324
Comparator using op-amp LM324

Explanation

  • Use LM324 op-amp for your experiment.
  • LM324 can be used with single rail power supply. i.e. only one battery with +ve and ground. It can be used with two power supply rails also if and when required.
  • Measure the voltage at the non-inverting terminal (pin 3). See the pin configuration of LM324 below)

  • Use the 10k preset to change the inverting input voltage value till the LED changes its state. Note this voltage. If this voltage is not equal to half of the power supply voltage, discuss the issue in the comments’ space.
  • Notice the voltage at pin 1. It is either zero or maximum. In our case, it will be near 7.2V not equal to the supply voltage 9V. This is because the V_{high} of the output stage of an op-amp is approximately 80% of supply voltage.
  • If you don’t face any difficulty in implementing the above ckt, Study the next article.

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