Week of October 29th

Week of October 29th - Class notes for the week of October...

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6 4 2 0 2 4 6 10 5 0 5 10 Vin Vout Class notes for the week of October 29 th Digital circuit input and output characteristics We have seen that operational amplifier circuits have a voltage transfer characteristic similar to that shown in Figure . Analog circuits, like the op amp are operated on the diagonal linear part of the voltage transfer curve. In this mode of operation, the output depends on the input voltage: if the input voltages changes even a little bit, the output voltage will also change. Op amps are optimized for good performance in this linear region, where good performance requires a balance of low noise, high gain, wide frequency response, and many other factors. We also observed that the output voltage was restricted by the voltage available from the power supply. If the input voltage increases beyond a certain voltage (2V in the figure), the output can not ‘keep up’ because it becomes limited by the available power supply voltage. When this happens, the circuit is said to be saturated , and we see the result as horizontal regions on the voltage transfer diagram. Op amps designers usually try to avoid operation in this region. Figure : Op amp voltage transfer characteristic Digital circuits, however, are optimized for operation in the saturated regions. This allows the digital designer to focus on one thing… speed. Digital circuits need not be precise, and they need not be low noise, they need to be fast. We will see that the digital circuits in our lab kits will be about 1000 times faster than the op amps in our kits. Because digital circuits are generally used with only a single power supply (as opposed to the split power supplies we have been using with our op amps), the voltage transfer diagram will appear slightly differently (Figure ). We observe that there are two horizontal parts on the diagram. In these horizontal regions, the output voltage remains
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0 1 2 3 4 5 0 1 2 3 4 5 Vin Vout essentially unchanged for fairly wide ranges of input voltages. We call the higher output voltage horizontal region the logic ‘1’ output state, and we call the lower output voltage horizontal region the logic ‘0’ output state. Thus for the circuit below we observe that the logic ‘1’ output voltage is 4.8V, and the logic ‘0’ output voltage is 0.2V. These voltages are important in logic design, so they have their own names, the higher output voltage is called V OH (voltage, output high) and the lower output voltage is called V OL (voltage, output low). These voltages will vary depending on the type (family) of logic circuits chosen by the designer. Figure : Digital circuit voltage transfer characteristic We see from the voltage transfer diagram that in addition to the logic ‘0’ and logic ‘1’ output states, there is a range of input voltages where the circuit will operate in the linear region. This ‘logic ½’ region is to be avoided at all costs in digital design, because the
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This note was uploaded on 09/26/2008 for the course ESE 123 taught by Professor Westerfield during the Fall '07 term at SUNY Stony Brook.

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Week of October 29th - Class notes for the week of October...

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