Building blocks of an Op-amp

Sheffield Hallam universityBuilding blocks of an Op-amp

Chathunika GamageEN12525904

ContentsTable of Figures1Abstract2Introduction2Background Theory2Discrete transistor differential amplifier long tailed pair2Current Mirror3Voltage level-shifting4Power output circuit4Experimental Method5Results7Discussion and Conclusion9Bibliography10

Table of Figures

Figure 1: Differential amplifier LTP(http://en.wikipedia.org/wiki/Differential_amplifier)2Figure 2: LTP with current mirror[3]3Figure 3: Level Shift circuit diagram[3]4Figure 4: V_BE multiplier [3]4Figure 5:Discrete differential amplifier long tail pair5Figure 6: LTP circuit with current mirror5Figure 7: Level shift circuit (Intermediate stage)6Figure 8: Complementary push-pull class AB circuit6Figure 9: LTP circuit7Figure 10: Simulation results of LTP7Figure 11: LTP with current mirror8Figure 12: Simulation results of LTP with current mirror8Figure 13: Simulation Result of Intermediate stage8Figure 14: Simulation Result of Complementary push-pull class AB circuit9Figure 15: 3 stage differential amplifier[3]9

Abstract

The internal circuitry of an operational amplifier is simulated on PSpice. There are certain building blocks which constitute an operational amplifier which are looked at in this lab. Simple theory is compared against simulation and sources of error are considered. Introduction

There are 3 main stages in the building blocks of an operational amplifier and they are mentioned below.

1) Differential amplifier (diff amp) input stage2) Intermediate amplifiers, with level shift + buffer3) Complementary (push-pull) output stage

Discrete transistor differential amplifiers long tailed pair, current mirrors as current sources, active loads and level shifting circuits are the main constituents of the operational amplifier.

Background TheoryDiscrete transistor differential amplifier long tailed pair

The input to the differential amplifier LTP is through where = = . This is the input stage of the differential amplifier. and are normally DC biased.

Figure 1: Differential amplifier LTP(http://en.wikipedia.org/wiki/Differential_amplifier)

In the op-amp, this circuit is made to have a single ended output at . A differential output could also be achieved by this circuit with double the gain. When the transistors are matched the current through as in figure 1 is twice the current through or .

Current Mirror

Current mirrors are extremely useful pieces of circuitry. Current mirrors consist of two matching transistors made from the same silicon die or bought from the same manufacturer. The collector of one of the transistors is connected to the bases of both the transistors.

Figure: A Current mirror[1]The configuration produces the same collector current on both transistors. The tail current of the differential amplifier LTP is provided in this case by the current mirror as shown in Figure 2.

Figure 2: LTP with current mirror[3]The common mode rejection ratio of the circuit in figure 2 is very high making sure unwanted input signals are rejected. This is due to the high effective impedance of the current mirror circuit. This allows the inputs to accommodate voltage swings without exceeding the active range of the transistors in the circuit. Which makes an important characteristic of using the current mirror.

Voltage level-shifting

A voltage level shifting circuit is used to adjust the output from the LTP circuit. What this means is that the voltage output from the input stage is not at 0V and needs to be shifted to 0V.

Figure 3: Level Shift circuit diagram[3]

When the output signal from LTP circuit is fed into the circuit in figure 3, the output at will be as close to 0V as possible. Only then will the circuitry in the next stage be at biasing point. The emitter follower transistor makes sure that the size of the single input to it is the same as the output but with a DC voltage shift. The characteristic unity gain of the emitter follower makes this possible. Power output circuit

The power output stage has the configuration of complementary (push-pull). This circuit primarily uses npn and pnp transistors. For the biasing of the two transistors the multiplier circuit is used.

Figure 4: V_BE multiplier [3]

The multiplier circuit play a significant role in getting rid of cross-over distortion in the op-amp and also helps in temperature compensation. This is why a class AB circuit is used here. Experimental Method

PSpice simulations were done to obtain typical circuit performance. The schematics of the circuits were drawn as shown in the diagrams below. Figure 5:Discrete differential amplifier long tail pair

Figure 6: LTP circuit with current mirror

Figure 7: Level shift circuit (Intermediate stage)

Figure 8: Complementary push-pull class AB circuit

ResultsThe simulation results of the LTP circuit is given below.

Figure 9: LTP circuit

Figure 10: Simulation results of LTPThe prediction shows that the tail current is twice that of the collector current.

This is in accordance with theoretical principles of the LTP circuit.The LTP circuit with the current mirror constitutes the input stage of op-amps. The simulation results of this circuit is given in figure 9 and figure 10.

Figure 11: LTP with current mirror

Figure 12: Simulation results of LTP with current mirrorThe tail current is now controlled by which is the current that passes through . The tail current is 5.53 mA and the reference current is 4.73mA.

Figure 13: Simulation Result of Intermediate stageThe voltage shifted is observed as expected.

Figure 14: Simulation Result of Complementary push-pull class AB circuitCross-over distortion is seen clearly on the output signal.Discussion and Conclusion

It is seen that the discrete differential amplifier long tail pair works as typically suggested by theory. The differential output will be 0V in this case and holds the theoretical equation This plays a significant role in the building blocks of the op-amp. This is depicted by figure 13. Figure 15: 3 stage differential amplifier[3]The current mirror circuit simulation prediction is as typically suggested by theory. Where the tail current is approximately equal to the reference current. In the prediction however it is seen that the tail current and the reference current has a percentage difference of 16.91%. This difference could be due to the fact that the two transistors used in the current mirror are not from the same manufacturer or not from the same silicon die. Current mirrors work best only when the transistors used are from the same die, side by side. Current mirror use is important as they provide high impedance in the input. Current mirror is also used in this case to provide an active load. This is a good thing because it provides high gain without giving a high load resistance. This also saves chip area as resistor are large relative to the rest of the circuitry. Bibliography1. B., R. & N., L., 2009. Electronic Devices and Circuit Theory. 7 ed. Columbus Ohio: PRENTICE HALL.2. Engineering, P. S. M. a. N., 2014. Inside the 741 op-amp. [Online] Available at: https://www.mne.psu.edu/me345/Lectures/Inside_the_741_Op_Amp.pdf3. Sheffield Hallam University, 2012. Operational Amplifier Building blocks Lab sheet. Sheffield: s.n.

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