Introduction - Facstaff Bucknell

advertisement
ELEC 350L
Electronics I Laboratory
Fall 2005
Lab #2: Inverting Amplifier with T-Bridge Feedback Network
Introduction
Op-amps are versatile electronic devices and are employed in a wide range of applications in
both analog and digital circuitry. One of these is high-gain analog amplification. The use of
negative feedback makes possible the design of stable high-gain amplifiers, which is a difficult
task with other electronic amplifying devices. Even so, care must be taken to limit the sensitivity
of the feedback network to environmental conditions. The T-bridge amplifier circuit was
developed to address this requirement. In this lab experiment you will build one of these
amplifiers and investigate its robustness with regard to changing environmental conditions.
Theory of Operation
To achieve high gain in a standard op-amp inverting amplifier, shown in Figure 1, the feedback
resistor R2 must have a very large value, especially if the input resistance (set by the resistor
connected to the inverting input of the op-amp) has to be relatively large. The use of large
resistors to control gain can pose a problem if there is stray resistance or capacitance in parallel
with the feedback resistor. Such stray impedances, which could be the result of dirt and/or
moisture, can lower the effective impedance of the feedback path. The problem can be avoided
by employing a “T” network made of resistors with smaller values. An inverting amplifier with
a T-bridge feedback network is shown in Figure 2.
R2
vin
R1
+15 V
_
s
d
+
vo
−15 V
RL
Figure 1. Standard op-amp nverting amplifier. The triangle symbols indicate
connections to circuit ground.
1
R2
RA
va
RB
vin
R1
+15 V
_
s
d
+
vo
−15 V
RL
Figure 2. Inverting amplifier with T-bridge feedback network. The triangle
symbols indicate connections to circuit ground.
The circuit is easily analyzed (as are most op-amp circuits) by using nodal analysis. At the
inverting input of the op-amp, application of Kirchhoff’s Current Law (KCL) yields
vin  0 0  va

,
R1
R2
which simplifies to
va  
R2
vin .
R1
Note that a virtual short is assumed to exist across the op-amp’s input terminals, which dictates
that the node voltage at the inverting input must be 0 V. At the node marked va, KCL leads to
0  va 0  va vo  va


 0,
R2
RB
RA
which becomes
R

R
vo   A  A  1va .
 R2 RB 
Substitution of the equation for va into the equation for vo yields directly the expression for the
voltage gain of the amplifier,
R
R
R
vo   A  A  1 2 vin .
 R2 RB  R1
Comparison of this gain formula to that for the simple inverting amplifier,
vo  
R2
vin ,
R1
illustrates that substantially higher gain can be achieved using smaller resistor values with the
T-bridge circuit than with the standard circuit.
2
Experimental Procedure
Design and assemble a standard inverting amplifier circuit like the one shown in Figure 1 using
an LM741 op-amp. The amplifier should have a gain of –60 ( 5%) and an input resistance of
150 k ( 5%). Use power supply voltages of ±15 V, and use nearest standard values for all
required resistor values. Let the load resistance (RL) be infinite (an open circuit) for now. Apply
a sinusoidal voltage at a frequency of approximately 1 kHz to the input of the op-amp. Note that
you will have to use a small-amplitude input signal to avoid the presence of clipping in the
output voltage waveform. (You might need to use a voltage divider like the one you investigated
in Lab #1.)


Once you have the circuit operating, touch the two ends of the feedback resistor with two
fingers from one hand and note any effects it has on the output voltage. What do you
think is causing the effects you see?
Next place your fingers across the input resistor. Do you see any effects? Why or why
not?
Now design and build the T-bridge circuit shown in Figure 2 to have a gain as close as possible
to –60 and an input resistance of 150 k. Again, use power supply voltages of ±15 V, leave RL
as an open circuit, and apply the same small-amplitude sinusoidal input signal at a frequency of
around 1 kHz.


Once again place two fingers from one hand across each of the three resistors in the
feedback network. Describe any effects you see, and compare them with those you
observed with the standard inverting amplifier. Highlight any notable differences.
Place your fingers across the input resistor. Do you see any significant effects this time?
Why or why not?
Connect a 220- load resistor (labeled RL in Figure 2) between the output of the T-network
inverter and ground, and adjust the input signal’s amplitude so that the output voltage has a value
of approximately 5 Vpk (10 Vpp).



Compare the output voltage waveform you observe with RL = 220  to that obtained with
no load resistor (a load of infinity).
Try two or three smaller values for the load resistor, down to just a few Ohms, and
describe (or plot) the output voltage waveform obtained in each case. Explain your
measurements in light of the information available on the LM741 data sheet. (Hint: The
Thévenin equivalent output resistance of the inverting amplifier is on the order of
fractions of an Ohm, so you are not observing a voltage-divider effect in a literal sense;
the effect you observe is caused by something else.)
For the smallest value of RL you used, explain whether or not the numerical value of the
peak output voltage is consistent with the information found on the LM741 data sheet.
A data sheet for the LM741 is available at the National Semiconductor web site at
http://www.national.com/ds/LM/LM741.pdf; a link is available on the ELEC 350 “Laboratory”
page. Remember to cite any information from the data sheet that you use.
3
Related documents
Download