Differential Amplifier Vs Op-amp | Parameters

Hello everyone, I hope you all are doing good in your life. However, before starting I would like to know that how many of you know that how a simple music system or the amplifiers at our home works? what is the principle behind its working? How it is possible to intensify the strength of the signal to such a level? and Moreover on what basis does the process take place? If you are not having the answers to the above question then don’t worry. Just continue to read and you will get to know all. Besides, you will also get to know differential amplifier Vs op-amp.

Differential Amplifier

A differential amplifier is a basic building block of an op-Amp i.e. we can say it is the core of an Op-amp. The main function of a differential amplifier is to amplify the difference between the two input signals.

V_o = V₁ – V₂

Depending on the number of inputs and also the way in which the output measures the differential amplifier can be configured as;

1. Dual input balance output
2. Dual input unbalance output
3. Single input balance output
4. And lastly the Single input unbalance output

Features of differential amplifier vs op-amp

1. Firstly it has a high differential gain and low common mode gain.
2. Secondly High common mode rejection ration and input impedance.
3. low output impedance.
4. lastly, and most importantly high gain.

further, we are going to know to understand the definitions and meaning of the terms mention above.

AC parameters of differential amplifier Vs op-amp

However, now let’s time to start to understand various ideal and non-ideal parameters of a differential amplifier

Differential Input signal

The difference between both the inputs is called a differential input signal. Moreover, V_d is use to represent it.

V_d = V₁– V₂

However, this will provide the output as per the applied input.

Common Mode Signal

A common-mode signal is a signal which is common to both the input terminals V1=V2=Vc. However, the output obtain by an ideal amplifier is equal to zero.

Common Mode Gain

Ideally common-mode gain should be equal to zero. However, in practice, the output voltage Vo of a differential amplifier depends on the differential signal or say the average of the two input signals known as Common Mode Signal[V_c]

V_c = (V₁ + V₂)/2

So the gain of differential amplifier with a common-mode signal is;

A_c = V_o/V_c

All in all, its practical value should be zero.

Differential Gain

However, differential Gain can be define as the ratio of output voltage(V_o) to the rate of change of differential input signal(V_d)

V_o = A_d * V_d

∴ A_d = V_o/V_d

Whereas in decibels it is express as;

A_d(dB) = 20log₁₀[V_o/V_d]

So from both the gain’s mention above we can get the total gain as,

V_o = A_d*V_d +A_c*V_c

CMMR (Common Mode Rejection Ratio)

As discuss above a common mode signal is present at both the input terminal, but these signals produce a very small nearly negligible output. Which in turn means that it is capable of rejecting the common-mode signal

Accordingly, it is define as the ratio Differential Gain (Ad) to the Common Mode Gain (Ac). Ideally, CMMR should be infinite Moreover practically it should be as high as possible.

CMMR = p = A_d/A_c

Whereas in decibel it is express as;

CMMR = 20log₁₀(A_d/A_c)

This is a very important characteristic of a differential amplifier because it represents the ability of the amplifier to suppress undesired disturbance like noise. as mention above the CMMR should be infinite so the common-mode gain must be as small as possible. In common mode operation, Ac depends on the value of Re. This resistance does introduce negative feedback and which eventually reduces the common-mode gain.

Higher the worth of Re, allot is the negative feedback and fewer is the common-mode gain. Thus increase in the Re decreases the Ac and CMRR increases.

However Q point depends on Re so if we increase Re to improve CMRR, the q point gets shifted. However, this is not acceptable. So we need to use some other method of improving CMRR

Methods to improve CMMR in differential amplifier vs Op-amp

1. Constant Current Bias
2. Current Mirror circuit
3. And lastly an Active load

Differential Input Resistance

Differential input resistances are outlining because the equivalent resistance will be measured at either input terminal with other terminal grounded. However, source resistance Rs1 and Rs2 are very small and hence will be ignored in the derivation of equivalent resistance.

R_i1 = R_i2 = 2β_ac.r_e

Output Resistance

Likewise, Output resistance is also define as the equivalent resistance that would be measure at the output terminal with respect to ground potential.

R_o1 = R_o2 = R_c = R_o

Op-Amp

An Op-Amp is basically a differential amplifier that amplifies the difference between two input. The block diagram and elements of a basic Op-Amp are;

1. Input Stage :- The input stage is a dual input differential amplifier. The two inverting inputs and the inverting input terminals provide most of the voltage gain and adjacent input resistance Ri.
2. Intermediate stage :- This is a dual input unbalance output differential amplifier (Because two inputs provide additional voltage gain)
3. Level shifting stage :- Due to direct coupling between the first two stage the dc voltage get amplified with non zero dc level shifting use to bring this dc level back to zero volts.
4. Output stage:- This is complementary output stage it provide increase in voltage gain and raise the current supplying capability of the Op-Amp it also ensures that the output is low.

Parameters of op-amp vs differential amplifier

Input Offset Voltage

It is the voltage that must be applied between the two input terminals of an Op-Amp to nullify the output.

V_io = V_dc1 – V_dc2

Input Offset Current

However the algebraic difference between the current into the inverting and non-inverting terminals.

I_io = | I_B1 – I_B2 |

Input Bias Current

Whereas Input bias current Ib is the average of current that flows into the inverting and non-inverting terminal of an Op-Amp.

I_B = (I_B1+I_B2)/2

Differential Input Resistance

Differential input resistance Ri is the equivalent resistance that can be measured at either inverting or non-inverting input terminal with other terminal connected to ground.

Output Resistance

On the other hand output resistance, Ro is the equivalent resistance that can be measured between the output terminal of Op-Amp and the ground terminal.

Supply Voltage Rejection Ration (SVRR)

The change in input offset voltage of Op-Amp because of the changes in the supply voltage rejection ration

SVRR = ▲V_io/▲V

Slew Rate

However, the slew rate is define as the maximum rate of change of output voltage per unit time period.

SR = dV_o/dt

Output Offset voltage

It is the output voltage due to input offset voltage and input bias current or input offset current.

When R_com is not connected;

V_OOT = (1 + R_f/R₁)V_io +R_f.I_io

When Rcom is connected;

V_OOT = (1+R_f/R₁)V_io +R_fI_iB

Thermal Drift

The average rate of amendment of input offset voltage per unit alter in temperature is termed as Thermal voltage drift. However, it is denoted by ▲Vio/▲T and expressed in μv/o_c

Whereas the average rate of change of input offset current or input bias current per unit change in temperature is called Thermal current drift. it is denoted by ▲Iio/▲T and ▲Ib/▲T and expressed in pa/°C

However, the maximum change in the time output offset voltage with respect to time is ;

E_v = V_OOT = [1+R_f/R₁] [V_ios/▲T]▲T + R_f[▲I_ios/▲T]▲T

V_o = (-R_f/R₁)V_in ± E_v

Conclusion

Furthermost here we are at the last part of the blog. I hope that I have helped you to get a clear-cut idea about the topic. even though if you are having any doubts or concerns then please feel free to ask down below. and also mention the part which you like the most. Besides, I would be really happy to know which topic you would like to read next on.

Have a nice day 🙂

Regards.

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