(Aktu Btech) Integrated Circuits Important Unit-1 The 741 IC Op-Amp

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Q1. Write a short note on 741 Op-Amp circuit and discuss the bias circuit.

Ans. A. Op-Amp circuit: 

• 1. Generally BJT based Op-Amp is focused on 741 Op-Amp circuit. If we use IC design, the circuits use large number of transistors but relatively few resistors and only one capacitor. 
• 2. The block diagram of typical Op-Amp is shown in Fig.

• 3. The equivalent circuit of Op-Amp is shown in Fig. 

B. Characteristics of ideal Op-Amp:  

• 1. Infinite voltage gain, A_v = ∞. 
• 2. Infinite input resistance, R_i = ∞. 
• 3. Zero output resistance, R_o = 0. 
• 4. Zero output voltage when input voltage is zero. 
• 5. Infinite bandwidth. 

C. Bias circuit: 

• 1. The bias circuit is used to provide proportional current in the collector of transistor. 
• 2. Some transistors form the current mirror, in which transistor collector provides bias current for other output stage of Op-Amp. 
• 3. Finally some transistors works to provide the V_BE drop between the bases of output transistors for proper functioning of the device. 

Q2. Describe what is mean by output short circuit protection and explain how it is achieved in the output stage of IC 741. 

Ans.

• 1. The short-circuit protection circuitry is shown in the Fig. 
• 2. The Op-Amp 741 contains a number of transistors that are normally in the OFF state. 
• 3. When the output terminal gets shorted to the ground, while keeping a positive output voltage due to certain input signal, this will induce a large current in the output transistor Q₁₄.
• 4. This will result in producing heat and cause burn-out of the transistor. Therefore, when current flow in Q₁₄ reaches 20 mA, voltage drop across R₆ becomes 27 x 20 = 540 mV, which bias the transistor Q₁₅.
• 5. Similarly, the maximum current in Q₂₀ is limited by R₇, Q₂₁ and Q₂₄.
• 6. When the current increases, the voltage drop across R₇ becomes sufficient and the transistor Q₂₁ becomes ON and Q₂₁ and Q₂₄ shunt the excess current away from the transistor Q₂₀, hence protects the output transistor.  

Output resistance of 741 Op-Amp: 

• 1. The output resistance of the Op-Amp R_out is determined from the Fig.  In accordance with the definition of R_out the input source feeding the output stage is grounded, but its resistance is included. 
• 2. We have assumed that the output voltage v_o is negative and thus Q₂₀ is conducting most of the current; transistor Q₁₄ has therefore been eliminated. The exact value of the output resistance will depend on which transistor (Q₁₄ or Q₂₀) is conducting and on the value of load current. 
• 3. The resistance of emitter of Q₂₃ is

This resistance appears in parallel with the series combination of r_o13A and the resistance Q₁₈-Q₁₉ network.

• 4. Since r_o13A alone is larger than R_o23, the effective resistance between the base of Q₂₀ and ground approximately to R_o23, the output resistance R_out as

• 5. The output resistance of the 741 is specified to be typically 75𝝮.

Q3. Determine the small-signal model of the second stage of the 741 Op-Amp.

Ans. 1. Fig. shows the 741 second stage Op-Amp prepared for small-signal analysis. Now we analyze the second stage to determine the values of the parameters of the equivalent circuit shown in Fig. 

a. Input resistance: The input resistance R_i2 can be found by 

Substituting the appropriate parameter values yields R_i2 = 4 M𝝮.

b. Transconductance: 

i. From the equivalent circuit of Fig. we see that the transconductance G_m2 is the ratio of the short-circuit output current to the input voltage.  

ii. Short-circuiting the output terminal of the second stage to ground makes the signal current through the output resistance of Q_13B zero, and the output short-circuit current becomes equal to the collector signal current of Q₁₇(i₁₇). This can be easily related to v_i2 as follows:

iii. These equations can be combined to obtain 

c.Output resistance:

i. To determine the output resistance R_o2 of the second stage in Fig. we ground the input terminal and find the resistance looking back into the output terminal. 

ii. So, R_o2 is given by

where R_o13B is the resistance looking into the collector Q1_3B while its base and emitter are connected to ground and R_o17 is the resistance looking into the collector of Q₁₇.

Q4. Draw and explain the simplified model of 741 Op-Amp.  

Ans. 1. Fig. shows a simplified model of the 741 Op-Amp. The gain of 2^nd stage is assumed sufficiently large. The output stage is assumed to be ideal unity-gain follower. 

2. From the Fig.

Q5. Find out the overall gain of an Op-Amp Ic-741 giving its cascaded equivalent circuit derived for its three stages. Also derive the relationship between f_t and slew rate for IC-741.

Ans. A. Overall gain of an Op-Amp IC-741:

The overall voltage gain A_v of the Op-Amp is the product of voltage gains of each stage as given by,  

B. Slew rate: The slew rate is defined as the maximum rate of change of output voltage per unit of time and is expressed in volts per microsecond, i.e.,  

C. Relationship between f_t and SR:  

1. Consider a voltage follower shown in Fig.(a). The input is large amplitude, high frequency sine wave.  

3. Fig.(b) shows the input-output waveform. 

4. The rate of change of the output is given by, 

Q6. Draw and explain the frequency response of IC 741. 

Ans. 1. The system employs the Miller compensation technique. 

2. A capacitor (C_c) of about 30 pF is connected in the negative feedback path of the second stage.  

3. An estimation of frequency of poles can be obtained as, 

Since, this value of capacitor is large so we neglect all other between capacitances the base and ground of transistor.

5. The total resistance, 

6. The dominant pole has a frequency f_p,

7. Calculate all the values of poles and a bode plot is shown in Fig. 

where, f_t is unity-gain bandwidth.  

9. Bode plot signifies that the phase shift at f_t is – 90° and thus margin phase is 90°. This phase margin is sufficient to provide stable operation of closed-loop amplifiers with any value of feedback factor 𝝱. 

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