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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 1va . 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