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Lab5_OpAmps_F08 - ECE 2100 Laboratory 6The Op-Amp...

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ECE 2100 Laboratory 6—The Op-Amp Monologues Introduction In these three-week laboratories we will use a common building block of circuit design called the operational amplifier or op-amp. This is an in- tegrated circuit consisting of numerous transistors and other elements. Its beauty is that, with two “golden” rules, fairly sophisticated linear circuit designs can be understood. The symbol for an op-amp is shown below. The most elementary device uses the five connections shown here: the positive (non-inverting) and negative (inverting) inputs are labelled V + and V ; the output is V o ; and the positive and negative power terminals are V CC and V EE , which is usually equal to - V CC . Notice that there is no specific terminal labelled ground. However, both of the power supplies and the circuit around the op-amp must be referenced to a common ground for the analysis to be valid. The very important golden rules for op-amps in the linear operating range are as follows: 1. By means of negative feedback (shown in the circuits on the following pages), the output voltage adjusts itself to force the two inputs to be almost the same voltage, i.e., for the ideal op-amp, V + = V . 2. The inputs ( V ± ) draw almost no current, i.e., for the ideal op-amp, I + = I = 0. For each experiment, using negative feedback, we will explain how to use these rules to analyze the circuit. The LF353 Dual Op-Amp We will use exclusively the LF353 dual op-amp in the labs and the design project as it is very easy to use and has excellent operational qualities. The pin assignments of the op-amp are drawn below. 1
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The supply voltage pins are common for both op-amps, and we would nor- mally connect these to ± 12 volts, using the power supply. In some exper- iments we may use only one of the op-amps. Although we treat op-amps as “ideal” when we design our circuit, there are real world limitations that must be kept in mind. All op-amps have a limited range of resistance values over which they will function as intended. For the LF353 you should work with resistances in the range from 2 kΩ to 4.7 MΩ. (From this point on, only units k and M will be used and omega for Ohms will be assumed.) If you go outside of this range, you run the risk of the op-amp not performing as you expect it should. The reason for this is that “ideal” op-amp golden rules are not exact; they are only approximations that make op-amp design easy. If the circuit resistors are too large, the fact that I + and I are not exactly zero will create a voltage that will affect the voltage difference at the op-amp inputs. If the resistors are too small, then the op-amp will draw too much current and will no longer be in its linear range. Another fact to keep in mind is that your op-amp circuit will not work properly if there is no dc path to ground (the ground of the power supply) in your circuit. Forgetting this point (which is easy to do since the supply connections are not shown explicitly) will likely lead to strange behavior by your circuit.
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