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Transistor as switch-op amp voltage amplifier?

  1. Jul 29, 2005 #1
    Hi can anyone explain to me how you can drive a transistor to function as a switch?what calculations i have to make in order to decide the type of transistor that i need and the kind of resistors that i need.

    also i have a voltage of maximum 80 mv.i want to enlarge it in order to drive the voltage to the transistor that i want to function as a switch.can i use op amp?i am reading a book and i am not sure i have anderstood their function.what kind of op amp i need?

    thanks
     
  2. jcsd
  3. Jul 30, 2005 #2

    berkeman

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    Staff: Mentor

    The most common way to use an NPN transistor as a low-side switch is to drive a current into its base with respect to the grounded emitter, and connect the load between your V+ supply and the collector of the NPN transistor. You would typically drive a voltage into the base through a resistor, and size the resistor so that you get enough base current to support the desired collector current. You will also usually put a resistor in series with the load and collector to limit the load current.

    When the transistor is on, you will get about 0.6V at the base, and the collector voltage will be determined by the load and the load resistor. If the transistor is driven hard enough to go into saturation, then the collector voltage can get as low as a couple hundred mV.

    I'd like to suggest that you check out the book "The Art of Electronics" by Horowitz and Hill. It's a really great intro book for electronics, and it has a very practical approach to understanding and designing circuits. Chapter 3 has some very good opamp info, for example, and the peak detector configurations shown in section 3.15 would be a typical way that you would do your input stage amplification and rectification, to make the signal that you would use to drive your NPN low-side switch.
     
  4. Jul 31, 2005 #3
    Just want to mention something. Berkeman said that a resistor is often put in series with the collector to limit current. While there is nothing wrong with limiting current in some cases, when a transistor is used as a switch to operate the coil of a relay for instance you would probably not use a current limiting resistor in the collector. You would select a relay that matches the power supply voltage on the circuit. The relay will only draw as much current as it was designed to if placed directly across the power supply. If this current is more than the base current * beta then there will be a significant voltage between the collector and emitter since the transistor is not 'turned on hard enough' to be able to sink the current needed by the relay. Naturally there needs to be a base resistor to limit current. It is usually selected in order to get a high enough base current so that even transistors with a poor beta will fully saturate with the specific load that is in the collector circuit.
     
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