Physics
Home > Physics > Options > The age of silicon > The Age of Silicon: 6. Amplifiers
9.9 Option - The Age of Silicon: 6. Amplifiers
Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 21 Aug 08]
describe the functions and the properties of an ideal amplifier
-
An ideal amplifier has:
- infinite input impedance, Zin → ∞ (any signal can be supplied to the op-amp without loading problems)
- zero output impedance, Zout → 0 (the power supplied by the op-amp is not limited)
- infinite bandwidth
- infinite (open loop) gain, Ao → ∞
- infinite bandwidth (another way of saying this is to refer to slew rate which is a measure of the rate at which the output voltage can change. Slew rate is measured in V/s which would be ∞ for an ideal amp)
- In an ideal amplifier the output voltage signal is an
exact copy of the input signal, only amplified
- In an ideal amplifier impedance is measured in ohms. Input impedance refers to how the circuit inputing to the amplifier sees its input. The output impedance refers to the ohm value of the device into which the amplifier will deliver its maximum output.
describe the characteristics of an operational amplifier
- An operational amplifier (op amp) is typically
constructed as an integrated circuit (IC). They come very
close to the ideal performance of an amplifier (see above).
Op amps are represented in circuits by the following
symbol:
-
Ideal op amps have the following characteristics:
- infinite input impedance
- zero output impedance
- infinite voltage gain
- infinite bandwidth
- zero input offset voltage.
-
By way of comparison the characteristics of the 741 which
is a real op amp are:
- the input impedance is about 2 Megohms
- the output impedance is about 75 ohms
- the voltage gain rolls off 6dB per octave starting at 100kHz
- the slew rate is 0.5V/microsecond
- there is a finite input offset voltage which must be zeroed by a resistor between pins 1 and 5. the input offset is typically 2mV to <6mV.
-
Note that op amps have two inputs, which can be selected
for particular purposes in real circuits. There are two
consequences of these characteristics for ideal op amps
that use external feedback. They are:
- the output does whatever is necessary to make the voltage difference between the inputs zero
- the inputs draw no current.
These consequences (called golden rules in the literature) allow us to design real op amp circuits (see summing amplifier described below).
explain
that the gain of an ideal amplifier is the ratio of its
output voltage to its input voltage:
- An amplifiers job is to amplify voltage, current or power. In this case the gain (or measure of the amplification) of a voltage amplifier is defined as the ratio of output voltage over the input voltage.
identify that an operational amplifier is a component of a typical amplifier
- Many typical amplifiers in use today have operational amplifiers as a component within their circuits.
distinguish between open-loop gain and closed-loop gain
- The gain of an amplifier can be specified in two ways.
Open-loop gain is the gain of the amplifier without a
feedback loop in the circuit and is the quoted gain from
manufacturers for the op amps they make.
- Many practical amplifiers use an external feedback loop
to ensure a stable operating environment for the amplifier
over the range of frequencies for which it is designed to
operate.
- The closed loop gain is usually much less than the open loop gain.
identify the voltage range over which an operational amplifier circuit acts as a linear device
Rf = feedback resistor
V+ = external supply to IC amp (in this case a 741)
V- = external supply to IC amp
1-8 = pin numbers on IC package
- By linear is meant that the output voltage is exactly proportional to the input voltage. To achieve this, the power supply voltages to the IC and output voltage it delivers need to be keep within this range: V- ≤ Vout ≤ V+
explain the difference between the non-inverting input and the inverting input
- The non-inverting input produces an output signal that is in phase with the input signal. The inverting input produces an output signal that is 1800 out of phase with the input. The inverting input is marked with a - on the IC circuit above and the non-inverting output is marked with a+.
describe how an operational amplifier can be used as an inverting amplifier
- An inverting amplifier produces an output voltage that is the inverse of the input voltage (if the input is positive, the output is negative). To be an inverting amplifier, the feedback loop needs to be connected to the inverting input (as shown in the diagram below).
explain
that the gain of an inverting amplifier is given by: 
- In the circuit above, R = Ri. This is the closed loop gain for the amplifier and the values of the two resistors are chosen to ensure that the output voltage keeps the IC within its linear operating region.
discuss how feedback can be used in a control system
- Feedback is used to ensure that op amps remain stable
and do what they are designed to do.
-
Negative feedback can be provided (as shown above) by
connecting a resistor between the output and inverting
input. The choice of feedback resistor controls the gain
of the amplifier so that it cant overload the next stage
in the circuit and it will keep the amplifier within its
linear operating range. Examples include:
- amplifiers (usually designed to operate within a set range of frequencies, such as the audio spectrum or one of the range of radio frequencies used to transmit radio, television and mobile phone signals)
- current to voltage converters as in a light detector circuit
- summing amplifier to make an audio mixer or a digital-to-analogue converter (see below)
- integrator
- differentiator
-
Positive feedback can be provided by connecting a
resistor between the output and non-inverting output.
Examples include:
- voltage follower circuit to buffer logic circuits
- oscillator circuits to make radio transmitters
gather information to identify the different ways in which amplifiers are used in current technologies
- Consult your own sources to identify specific examples
of technologies that use amplifers in the ways listed
above. Feed the search terms op amp amplifiers and
uses into an appropriate Internet search engine
and identify the ways amplifiers are used in a range of
current technologies.
- Record the results of your research in an appropriate way.
explain the use of two input resistors to produce a summing amplifier
- The input resistors are chosen to provide a combined
input current that matches and cancels the current in the
feedback resistor (at point A) thus preserving the first of
the golden rules for ideal op amps using external feedback.
( I = V / R )
V 1 / R 1 + V 2 / R 2 = - V out / R 3
gather and present graphical information to show the transfer characteristics of an inverting amplifier
- The transfer characteristics of amplifiers are
determined from data gathered by monitoring and recording
data from operating amplifier circuits. The goal is to
optimise the circuit performance so that the output signal
is an exact copy of the input signal shape (ie the
amplifier is operating in its linear region for the input
and output voltages you want to use).
- Below is a graph showing the relationship between the output voltage of an inverting amplifier and its corresponding input signal when it is operating in its linear region.
- You might like to see if you can find data from
manufacturers of op amps from which to construct graphs
like that above.
- See notes related to the next two syllabus points
solve problems and analyse to show the transfer characteristics of an amplifier
- See notes related to the next syllabus point
solve
problems and analyse
information about setting the gain of an inverting
amplifier by calculating the values of external resistors
using:
-
Q 32 (c) parts (i), (ii) and (iii)
from the 2002 HSC Physics exam. Scroll down to p 37
(the last page)
-
Q32 (d) part (i) and (ii) from the
2003 HSC Physics exam. Scroll down to p 42.
-
Q32 (d) parts (i), (ii) and (iii) from
the 2004 HSC Physics exam. Scroll down to p 38.
perform a first-hand investigation of a summing amplifier by adding voltages from two separate sources
- You will need to do this first hand investigation but
it is important that you discuss it with your teacher to
make sure it is done properly and safely and that your
school has the equipment needed.
- You could record your results in a table.
|