Amplifier Basics A small signal is amplified to a large signal Gain is determined by the function of Vout/Vin or Iout/Iin or Pout/Pin Most amplifiers

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1 Op Amps

2 Amplifier Basics A small signal is amplified to a large signal Gain is determined by the function of Vout/Vin or Iout/Iin or Pout/Pin Most amplifiers are frequency specific i.e. they only operate for a certain set of frequencies Amplifiers take a signal from the source and introduce a small load to the source Amplifiers have to provide signal for the output to drive another circuit

3 Key terms Amplifiers Bandwidth measured in F Gain a factor Input impedance - Zin Output impedance Zout Power dissipation Common Mode Rejection Ratio Noise SNR

4 Ideal Amplifier Infinite gain Infinite bandwidth 0 ohms output impedance Infinite input impedance Infinite common mode rejection ratio 0 noise 0 losses i.e. 100% efficient

5 Op Amps Made originally for analogue computing Widely used for instrumentation, bespoke circuits, small amps, buffer circuits Cheap technology Readily available Robust Versatile

6 Op Amps Dual Power supply +15V and -15V Zin 2Mohms (note: may change with resistive networks) Zout 75ohms Gain 200,000 Bandwidth 1MHz at unity gain realistically, 10 to 100kHz bandwidth forced down due to internal cct.

7 Op Amps Only few components needed to generate precise gain

8 The Op Amp IC Typical numbers 741, 747, LM324, LM 339 Packages come in single, dual and quad op amps, in DIL 8 pins and 14 pins

9 How to kill an Op Amp

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13 LM 339

14 LM358

15 Op Amp Golden Rules 1. The output tries to do whatever is necessary to make the voltage difference between the two inputs zero 2. The two inputs do not draw any current

16 OP Amp Circuit

17 OP Amp Packaging

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21 OP Amp Inverting Amplifier

22 Inverting Amplifier Note: that the output voltage is inverted i.e. positive Voltage is applied at the input a negative Voltage will be seen at the output

23 Task Inverting Amplifiers Having a box of 10k resistors and 741 op amps design an amplifier having a gain of 10 Design a circuit which can be used as a buffer.

24 OP Amp Non Inverting Amplifier

25 Task Non Inverting Amplifier Having a box of 10k resistors and 741 op amps design an amplifier having a gain of 9 How could you design a non-inverting amplifier having a variable or tunable gain?

26 Task: You have got a box full of 10k resistors, hundreds of them. You have a dynamic microphone which you want to use for a PA system. Unfortunately the PA system has only got "Line" level input. Using diagram below work out what a typical line level input is:

27 A dynamic microphone has a typical input level of 1.5 millivolts RMS. Work out: - peak voltage for a typical dynamic microphone - how much gain do you need to generate to achieve 0dBu Ref level (pro)? - Which Op-Amp design would you choose Non-Inverting or Inverting, what will be the difference - What will be the significance if the input impedance is low? - Design an Op Amp to reach line level voltages - How would you use your 10k resistors to get the desired values - Series or Parallel connections - using Multisims verify your findings - with line level input impedance of 10k, would you require a voltage follower? Discuss?

28 Differential Amplifier

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30 Differential Amplifier Formula Note: R1 = Rf = R2 = Rg will not generate any gain but only amplify the difference. Using a different ratio for R1 and Rf or R2 and Rg will introduce a gain factor in the output

31 Differential Amplifiers Tasks: 1. Calculate the output using the formula above: V1 = 5V V2 = 2V All resistors have a value of 10k 2. Calculate the output: Using the same voltages as above but with different resistors R1 = 10k R2 = 10k Rg = 20k Rf = 20k What do you find?

32 Voltage Follower (unity gain amplifier or unity buffer amplifier) Realistic value for Z is about 1 Mohm to 1 Tohm. The circuit is used to minimise impact on the source where Vin comes from and to provide higher current at Vout without impacting the source circuit. Nevertheless the circuit may become unstable when connected to a capacitive load. A solution may be to use two inverting amplifiers configuration with R1 and Rf of equal value.

33 What does this circuit do?

34 The Summing Amplifier

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36 How does the circuit work? Think about the Op Amp rules

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38 The Inverting Integrator What does the circuit do?

39 The Inverting Integrator Remember: Capacitors like a.c. but hate DC An ac signal can easily pass through a capacitor Remember that the higher the frequency the lower the reactance Xc measured in ohms. The circuit works as a low pass filter! It allows low Hz to go through freely and higher frequencies are cut out hence the term Top Cut Filter.

40 Fun with OP Amps

41 The Inverting Integrator A pointer: RC is the time constant, it will give the time it takes for a signal to reach it's peak. 95% of the peak is achieved after three time constants.

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44 Line following cct. Using Op Amps