EE 2B1 – Analogue Electronics Dr. T. Collins

Course Contents Analogue Electronics Operational Amplifiers Non-ideal behaviour Non-linear applications Transistor Amplifiers Review of the common-emitter amplifier Differential amplifier Multi-stage amplifiers Solid State Devices Second half of term, Dr. Childs

Content Delivery Lectures Define course content Explain key concepts Course Notes Contains everything you need to understand You may need a text-book in order to actually understand it though (Sedra & Smith recommended) On-Line Material Some circuit analysis walkthroughs PowerPoint slides Frequently Asked Questions

Assessed Sessions Tutorials Use these to ask questions! Laboratory Experiments Use these to ask questions too! Practical experience Simulated circuit analysis also Exams Example questions to follow…

Operational Amplifiers Properties of the ideal op-amp Revision of 1 st year Linear amplification circuits More revision Effects of non-ideal properties Non-linear applications Comparators Oscillators Precision rectification More…

Ideal Op-Amp Properties Open-loop gain, A 0, is infinite Input impedance is infinite Input current = 0 Output impedance is zero

Negative Feedback Actually, the open-loop gain is finite but not accurately known Open-loop gain varies: from device to device at different frequencies with temperature We usually want predictable performance from an amplifier Solution: negative feedback

Negative Feedback in Everyday Life Listening to the radio/tv If the sound is too loud: turn the volume down If the sound is too soft: turn the volume up What are we doing... Measuring the sound level (using our ears) Comparing with the desired target level Error = Desired level – Measured level Adjust volume by an amount proportional to error Result: We don’t need to know the exact relationship between the volume knob and the sound level in order to get the desired output.

In Technical Terms… Aim of the circuit is to minimise the error.

Simple Example Output > Input  Error is -ve  Output goes down Output < Input  Error is +ve  Output goes up Output  Input  Error  0  Output is unchanged i.e. System is in a state of equilibrium

Confused? If Output  Input  Error  0, then shouldn’t the output be zero? Yes, unless the amplifier gain is infinite. In practice, error is a very small number (  0 for the purpose of calculations). Equilibrium state is when the output approximately equals the input.

Buffer Amplifier but

Buffer Amplifier Examples Output should equal input? (In practice, A 0  10 5 so error is tens of microvolts)

Common Assumptions In any linear op-amp circuit, if the open-loop gain is assumed to be infinite, the difference between the inputs (the error) must be zero to give a finite output. Also, op-amps are designed to have a very high input impedance (ideally infinite) Common assumptions are therefore: The input voltages are equal The input currents are zero The output voltage will tend to the required level in order to satisfy the other two conditions (thanks to negative feedback)

Applying the Assumptions Given: From (2), V OUT = V IN From (1), R IN =  From (3), R OUT = 0

Buffer Amplifier Summary As long as the open-loop gain is big the output voltage will approximately equal the input. The exact value of A 0 doesn’t matter. The output impedance will be zero. The input impedance will be infinite. (ideally) (in practice, A 0 might not be very big, input impedance is finite and output impedance is non- zero… more on the resulting effects later)

Negative Feedback Summary If the response of a system is unknown, negative feedback can allow the output to be controlled. The circuit/system aims to minimise the error signal. If the gain of the system is infinite, the error can be reduced to zero. NB. A feedback network can be introduced if a gain other than one is required.

Next Time More revision of linear op-amp applications Non-Inverting Amplifier Inverting Amplifier Summing Amplifier Integrators Difference Amplifier All still assuming an ideal op-amp… for now.