1. Amplification and Feedback
ESS205
Spring 2004

2. How stable should frequency be as voltage is changed?
LM555 datasheet – 0.3% per volt, typical

3. Frequency stability vs. voltage
Actual components have many non-ideal characteristics
Temperature coefficients, voltage coefficients, memory effects, drift…
Specifications matter!
Disk
ceramic
Monolithic
Mylar film

4. 555 lab followup
Stability of oscillation frequency as power supply voltage is varied
Erratic measurement of frequency with multimeters

5. Frequency Meters
Need AC voltage > 250 mV RMS to determine frequency
Some show sensitivity to voltage “spikes”
Putting resistor in series with meter improves performance – low pass filter effect
Bad reading
Good reading

6. Amplifiers Make a signal (voltage or current) bigger
Voltage amplifier: Voltage out = Voltage in x G (gain)
Small signals: Sensor outputs, radio reception, microelectronics
Big signals: Provide mechanical energy (motors, speakers), radio transmission, heating

7. The ideal amplifier G
Vin
Vout = G x Vin

8. Amplification devices
Mechanism in which a small signal controls a large signal
Water analogy: turning the faucet (small signal) controls large flow of water (big signal)

9. Vacuum tubes Electrons “boil” off of heated cathode
Voltage on grid(s) control current reaching anode
Anode
Cathode
Heater
electrons

10. Transistor
Invented 1947
Made of semiconductors – silicon, germanium, gallium arsenide
Layered structure – creates junctions
The first transistor –
Bell Labs, 1947

11. Transistors
Voltage between base and emitter controls current between collector and emitter
Types: bipolar (NPN, PNP), JFET, MOSFET
~ 1 mm

12. Approximate transistor behavior – the Ebers-Moll equations
Explicit temperature dependence
Parameters (aF, aR, IES, ICS) dependent upon manufacturing details, temperature, etc.

13. Simple device – complicated behavior
The physical laws which describe electronic behavior are not simple
Want to make electronic components “act” simple to make them easier to apply
Need to build a complicated device to get simple behavior!

14. How to make electronics “act” simple?
Emphasize use of components which have close to ideal behavior
Resistors, capacitors
Use circuits which are inherently self-compensating for nonideal behavior
Ex: voltage divider

15. Modern operational amplifiers
~ 1 mm
LT1006
LMH6642

16. What’s a good “building block” for amplification?
One option: Fixed gains (10, 100, etc)
Instrumentation amplifier
Another option: Make G…really big, then “throw away” gain you don’t need G
Vin
Vout = G x Vin

17. Operational amplifiersV+
Vout=G x (V+-V-) V-
G very large (AD820: G ~= 1 million (DC, low load))
Inputs draw little current (AD820: ~10 pA)

18. Feedback amplifier Feedback: output “feeds back” to input
Op amps are always used with feedback
How does this work? G
Noninverting
Feedback
Amplifier

19. Rationale for use of feedback amplifiers
Amplifier performance is dictated largely by the behavior of simple passive components (e.g. resistors, capacitors)
Pioneers: H. S. Black, H. Nyquist, H. W. Bode, Ball Laboratories, 1920’s
“Black’s patent application was delayed for more than nine years in part because the concept was so contrary to established beliefs that the Patent Office initially did not believe it would work. They treated the application in the same manner as one for a perpetual motion machine.” -- W. M. Siebert, Signals and Systems

20. Basic op amp configurations
Noninverting
Inverting

21. Quick analysis of op amp circuits
Because G is so large, the difference between V+ and V- must be very small
To a good approximation V+=V-
Ex: V- is the output of a voltage divider: V- = Vout * R2/(R1+R2). V+ = V-, so Vout = V+ (R1+R2)/R2.

22. Other op amp configurations
Endless variety of circuits for performing different functions: filtering (high pass, low pass, band pass, band reject), integration, differentiation, calculating logarithms, square roots, etc.
Use different feedback components and interconnections
Ex: Sallen-key lowpass filter

23. Buffer amplifier
Gain of 1 – why use it at all?

24. Output impedance Ideal voltage source: Voltage independent of load
Real voltage source: Acts like an ideal voltage source in series with a resistor
Small resistance = “low output impedance”
Ideal
Real

25. Ex: Voltage divider No buffer amplifier: Vout depends on RL
With buffer amplifier: Vout
Independent of RL

26. Buffer amplifier – gain of 1
Buffer amplifier decreases the output impedance of a signal
Makes it easier to transmit signals between parts of a circuit – enables modular construction

27. Op amp packaging Generally 1, 2, or 4 to a package Standard pinouts
Single
Dual
Quad

28. Other op amp parameters
Speed (slew rate, bandwidth) – what type of signal can be amplified
DC, audio, video
Precision (input offset voltage) – how closely does the op amp equalize its inputs
Power (supply current, output current) – how much power does the amplifier require, and how much power can it deliver to a load

29. Input and output range
Op amp can only amplify signals within a range set by its power supply – can limit applications
“Single supply” op amps have input and output voltage range extending to the negative supply
Dual supply
Single supply