Lab 6
ENGR 2105 Amplifiers
Dr. Kory Goldammer
Amplifiers
An amplifier increases the voltage (amplitude) of an electronic signal, AC or DC, as shown in the figure above.
Amplifiers are found everywhere – in mobile phones, TV’s, radios. MP3 players, small appliances, cars, etc.
Most amplifiers are linear. That is, the output is a constant (K, called the gain) times the input (K>>1).
The Operational Amplifier (“Op Amp”)
The “op amp” typically has 5 inputs:
Two Inputs: ±V (normally equal magnitude + and ‒ voltages; the power inputs). We will use the 741 Op Amp and . This is the Maximum/Minimum voltage that can be output by the Op Amp
Two inputs (+ and -)
One Output.
There is no ground or common terminal, although one is usually supply supplied by the input power supply
The 741 Op Amp Chip Pin-Outs
Pin 2 is the negative input and pin 3 is the positive input
Pin 6 is the output signal
Pin 7 is where we apply 15 V, and pin 4 is where we apply -15V
Pins 1, 5, and 8 are not connected to anything
Amplifier Gain: K
The gain, K, of the Ouput is the amount of amplification. K is a real number.
The output of most amplifiers is linear:
The Gain changes only the amplitude of the signal, so the output signal “looks like” the input signal but with a larger amplitude
Characteristics of the Op Amp (1)
The Op Amp shown here is configured as “Open Loop”.
“Open Loop” Op Amps have very high gain (K~100,000)
Such high gain is not useful. We will use a configuration known as “Negative Feedback”
Characteristics of the Op Amp (2)
The output of the Amplifier cannot exceed
Any voltage that theoretically exceeds will have the peaks “clipped”, and the output signal will be “distorted”
Negative Feedback
To get a useful value for the Gain, we use Negative Feedback
This technique feeds back a portion of the output signal to the negative input
The feedback allows control of the amount of gain that the circuit provides.
Kirchoff’s Current Law (KCL)
Kirchoff’s Current Law
The sum of the currents entering a junction must equal the sum of of the current exiting a junction.
Negative Feedback Circuit Analysis (1)
Using KCL at Node n:
Op Amps are Designed with a very high input impedance, so we can approximate In = 0, which means
Negative Feedback Circuit Analysis (2)
From Ohm’s Law:
Using these three equations and rearranging:
Negative Feedback Circuit Analysis (2)
From Ohm’s Law:
Using these three equations and rearranging:
=
Negative Feedback Circuit Analysis (3)
So, we can specify the gain simply by altering the ratio of the feedback resistor, Rf, and Input resistor, Ri. Note that K is negative!
=
-K: What this means
You should note in the expression for K that we developed is a negative number.
This means that your amplifier inverts the signal while amplifying it.
Thus if the input signal is some small DC voltage V, then the output is ‒KV, K times larger than V, but of the opposite sign.
If the input is a sinusoidal AC signal v(t), then the output will be
-Kv(t); it will be 180° out of phase with the input signal. When the AC signal is rising above 0, the output will be falling below 0.
In a single-stage (one amplifier only) negative feedback amplifier,
the output is always inverted compared to the input signal.
Non-Inverting Amplification
What if we don’t want an inverted signal?
For the negative-feedback amplifier circuit, a single amplifier will always invert the sense of the output (with relation to input).
However, op amp circuits are easy and convenient to cascade, that is, to connect in series to increase amplification while keeping K manageable.
Regardless of connecting amplifiers in series, each one still inverts the signal between input and output.
Use this information for the last part of your experimental exercises.