Differential Amplifier and Analysis (LICA First Unit Notes)

Topic Wise Notes

Differential Amplifier- DC and AC analysis of Dual input Balanced output Configuration

Properties of other Differential Amplifier Configuration

Dual Input Unbalanced Output

Single Ended Input – Balanced/ Unbalanced Output

DC Coupling

Cascade Differential Amplifier Stages

Level translator

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Operational Amplifiers and Differential Amplifiers

Operational Amplifiers:

Basic Concept Operational Amplifiers:

Operational Amplifiers are used in Electronic Circuit Boards, Operational Amplifiers are Electronic Devices, which take a relatively weak signal as an input (like Inverting and Non-Inverting) and produce a much stronger (Amplified) signal as an output. The operational amplifier (Op-amp) is a special kind of amplifier used in Equipment such as stereo equipment, Robotics (which is takes IR signals) and medical cardiographs (which amplify the heart beat).

Principal of introduction and operation of Operational Amplifier:

Operational Amplifier is multi terminal device which is quite complex. Fortunately for the ordinary user, it is not necessary to know internal makeup.

An operational amplifier, or op-amp, is a very high gain differential amplifier with high input impedance and low output impedance. Typical uses of the operational amplifier are to provide voltage amplitude changes (amplitude and polarity), oscillators, filter circuits, and many types of instrumentation circuits. An op-amp contains a number of differential amplifier stages to achieve a very high voltage gain.

Following figure shows a basic op-amp with two inputs and one output as would result using a differential amplifier input stage. The terminal with a (+) sign is called Non- Inverting input terminal and the terminal with a (-) is called Inverting input terminal. Which produces a much stronger (Amplified) signal as an output while different inputs. The inverting signals input is the ac signal (or dc) applied to the differential amplifier. This produces 180 degrees out of phase signal at the output.  

Study the physical Diagrammatically phenomena of Operational Amplifier:

In this internal block diagram we can observe various stages, 

Stage 1: Inverting Input and Non- Inverting Inputs

Stage 2: Input Stage- Dual Input Balanced Output Differential Amplifier and Intermediate Stage – Dual Input Unbalanced Output Differential Amplifier.

Stage 3:  Level Shifting Stage – Emitter Follower using constant current source.

Stage 4 : Output Stage – Complementary Symmetry Push-Pull Amplifier  

The inverting and non-inverting inputs are provided to the input stage which is a dual input, balanced output differential amplifier. The voltage gain required for the amplifier is provided in this stage along with the input resistance for the op-amp.  The output of the initial stage is given to the intermediate stage, which is driven by the output of the input stage.

In this stage direct coupling is used, which makes the dc voltage at the output of the intermediate stage above ground potential.

Therefore, the dc level at its output must be shifted down to 0 Volts with respect to the ground.  For this, the level shifting stage is used where usually an emitter follower with the constant current source is applied.

The level shifted signal is then given to the output stage where a push-pull amplifier increases the output voltage swing of the signal and also increases the current supplying capability of the op-amp.

The operational amplifier is a direct-coupled high gain amplifier usable from 0 to over 1MHZ to which feedback is added to control its overall response characteristic i.e. gain and bandwidth. The op-amp exhibits the gain down to zero frequency.

Such direct coupled (dc) amplifiers do not use blocking (coupling and by pass) capacitors since these would reduce the amplification to zero at zero frequency. Large by pass capacitors may be used but it is not possible to fabricate large capacitors on a IC chip. The capacitors fabricated are usually less than 20 pf. Transistor, diodes and resistors are also fabricated on the same chip.

Differential Amplifiers:

Differential amplifier is a basic building block of an op-amp. The function of a differential amplifier is to amplify the difference between two input signals.

How the differential amplifier is developed? Let us consider two emitter-biased circuits as shown in fig. 1.

Fig. 1

The two transistors Q₁ and Q₂ have identical characteristics. The resistances of the circuits are equal, i.e. R_E1 = R _E2, R_C1 = R _C2and the magnitude of +V_CC is equal to the magnitude of –V_EE. These voltages are measured with respect to ground.

To make a differential amplifier, the two circuits are connected as shown in fig. 1. The two +V_CC and –V_EE supply terminals are made common because they are same. The two emitters are also connected and the parallel combination of R_E1 and R_E2 is replaced by a resistance R_E. The two input signals v₁ & v₂ are applied at the base of Q₁ and at the base of Q₂. The output voltage is taken between two collectors. The collector resistances are equal and therefore denoted by R_C = R_C1 = R_C2.

Ideally, the output voltage is zero when the two inputs are equal. When v₁ is greater then v₂ the output voltage with the polarity shown appears. When v₁ is less than v₂, the output voltage has the opposite polarity.

The differential amplifiers are of different configurations.

The four differential amplifier configurations are following:

1.      Dual input, balanced output differential amplifier.

2.      Dual input, unbalanced output differential amplifier.

3.      Single input balanced output differential amplifier.

4.      Single input unbalanced output differential amplifier.

Fig. 2

These configurations are shown in fig. 2, and are defined by number of input signals used and the way an output voltage is measured. If use two input signals, the configuration is said to be dual input, otherwise it is a single input configuration. On the other hand, if the output voltage is measured between two collectors, it is referred to as a balanced output because both the collectors are at the same dc potential w.r.t. ground. If the output is measured at one of the collectors w.r.t. ground, the configuration is called an unbalanced output.

A multistage amplifier with a desired gain can be obtained using direct connection between successive stages of differential amplifiers. The advantage of direct coupling is that it removes the lower cut off frequency imposed by the coupling capacitors, and they are therefore, capable of amplifying dc as well as ac input signals.

Dual Input, Balanced Output Differential Amplifier:

The circuit is shown in fig. 1, v₁ and v₂ are the two inputs, applied to the bases of Q₁ and Q₂ transistors. The output voltage is measured between the two collectors C₁ and C₂ , which are at same dc potentials.

D.C. Analysis:

To obtain the operating point (I_CC and V_CEQ) for differential amplifier dc equivalent circuit is drawn by reducing the input voltages v₁and v₂ to zero as shown in fig. 3.

Fig. 3

The internal resistances of the input signals are denoted by R_S because R_S1= R_S2. Since both emitter biased sections of the different amplifier are symmetrical in all respects, therefore, the operating point for only one section need to be determined. The same values of I_CQ and V_CEQ can be used for second transistor Q₂.

Applying KVL to the base emitter loop of the transistor Q₁.

The value of R_E sets up the emitter current in transistors Q₁ and Q₂ for a given value of V_EE. The emitter current in Q₁ and Q₂ are independent of collector resistance R_C.

The voltage at the emitter of Q₁ is approximately equal to -V_BE if the voltage drop across R is negligible. Knowing the value of I_C the voltage at the collector V_Cis given by

          V_C =V_CC – I_C R_C

and V_CE = V_C – V_E

               = V_CC – I_C R_C + V_BE

       V_CE = V_CC + V_BE – I_CR_C       (E-2)

From the two equations V_CEQ and I_CQ can be determined. This dc analysis applicable for all types of differential amplifier.

Example - 1

The following specifications are given for the dual input, balanced-output differential amplifier of fig.1: 
R_C = 2.2 kΩ, R_B = 4.7 kΩ, R_{in 1} = R_{in 2} = 50 Ω , +V_CC = 10V, -V_EE = -10 V, β_dc =100 and V_BE = 0.715V.
Determine the operating points (I_CQ and V_CEQ) of the two transistors.

Solution:

The value of I_CQ can be obtained from equation (E-1).

The values of I_CQ and V_CEQ are same for both the transistors.

Dual Input, Balanced Output Difference Amplifier:

The circuit is shown in fig. 1 v₁ and v₂ are the two inputs, applied to the bases of Q₁ and Q₂ transistors. The output voltage is measured between the two collectors C₁ and C₂, which are at same dc potentials.

Fig. 1