Active loads in amplifier circuits This worksheet and all related … Question5 An interesting technique to achieve extremely high voltage gain from a single-stage transistor amplifier is … This worksheet and all related files are licensed under the Creative Commons Attribution License, Active loads in amplifier circuits This worksheet and all related files are licensed under the Creative Commons Attribution License, version1.0. To view a copy of this license, visit http://creativecommons.org/licenses/by/1.0/, or senda letter to …

Question3 Suppose we were to compare the performance of two voltage divider circuits side-by-side. The circuit on the left has one variable resistor (R 2 ), while the circuit on the right has two variable resistors (R 1 and R 2 ). The right-hand circuit’sresistorsare ganged together in such away that as one resistance increases, the other will decrease by the same amount, keeping the circuit’stotalresistance constant: R 1 R 2 V out R 1 R 2 V out R total varies R total remains constant Knowing that the voltage output by a voltage divider is described by the following formula, determine which voltage divider circuit yields the greatest change in output voltage fora given change in R 2 ’sresistance. V out =V battery * R 2 R 1 +R 2 Question4 The purpose of a current mirror circuit is to maintain constant current through a load despite changes in that load’sresistance: Load I constant If we were to crudely model the transistor’sbehaviorasan automatically-varied rheostat -constantly adjusting resistance as necessary to keep load current constant-how would you describe this rheostat’s response to changes in load resistance? Load I constant . . . R transistor In other words, as R load increases, what does R transistor do-increase resistance, decrease resistance, or remain the same resistance it was before? How does the changing value of R transistor affect total circuit resistance? file 02656…. Notes4 This model of current mirror transistor behavior, albeit crude, serves as a good introduction to the subject of active loads in transistor amplifier circuits. This is where a transistor is configured to operate asa constant-current regulator, then placed in series with an amplifying transistor to yield much greater voltage gains than what is possible with a passive (fixed resistor) load. Notes5 There is more than one way to comprehend this effect, and why it works as it does. One of the more sophisticated ways is to consider what the internal resistance of a perfect current source is: infinite. Ask your students how they contemplated this effect, and what means they employed to grasp the concept. Notes6 Even if students do not yet know what an”operational amplifier”circuit is, they should still be able to identify transistor stages, configurations, and active loads. In this case, most of the active loads are obvious (asrevealedbythe current source symbols). Don’tbesurprisedif some of your students point out that the differential pair in this opampcircuit looks “upside-down”compared to what they’veseenbeforefor differential pair circuits. Let them know that this is not really an issue, and that the differential pair works the same in this configuration. Notes7 Even if students do not yet know what an”operational amplifier”circuit is, they should still be able to identify transistor stages, configurations, and active loads.

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