Transistor as switch-op amp voltage amplifier?

In summary, to use an NPN transistor as a low-side switch, you would drive a current into its base and connect the load between the V+ supply and collector. A resistor is usually placed in series with the load and collector to limit current. It is important to drive the transistor hard enough to go into saturation for lower collector voltage. For more information, you can refer to "The Art of Electronics" by Horowitz and Hill, which has practical information on op amps and circuits. When using a transistor to operate a relay, it is not necessary to use a current limiting resistor in the collector, but a base resistor is needed to ensure full saturation.
  • #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?

Engineering news on
  • #2
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.
  • #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.

1. How does a transistor function as a switch?

A transistor can function as a switch by controlling the flow of current between its collector and emitter terminals. By applying a small amount of current at the base terminal, the transistor amplifies it and allows a larger amount of current to flow from the collector to the emitter, effectively turning the switch "on". When no current is applied at the base terminal, the transistor blocks the flow of current and turns the switch "off".

2. What is an op amp and how does it relate to a transistor?

An op amp, short for operational amplifier, is a type of integrated circuit that uses multiple transistors and other components to amplify an input signal. It typically has two inputs and one output, and can be configured to perform various mathematical operations on the input signal. A transistor is often used within an op amp to amplify the signal, as it can provide a high gain and low output impedance.

3. How can a transistor be used as a voltage amplifier?

A transistor can be used as a voltage amplifier by connecting it in a common emitter configuration. In this setup, the input voltage is applied to the base terminal and the output voltage is taken from the collector terminal. The transistor amplifies the input voltage, with the amount of amplification determined by the resistance values in the circuit. This allows the output voltage to be much larger than the input voltage.

4. What are some advantages of using a transistor as a switch or op amp voltage amplifier?

Using a transistor as a switch or op amp voltage amplifier offers several advantages, including high efficiency, low cost, and compact size. Additionally, transistors have no moving parts, making them more reliable and durable compared to mechanical switches. They also have a fast response time, making them suitable for high-speed switching applications.

5. What are some common applications of a transistor as a switch or op amp voltage amplifier?

Transistors are commonly used as switches in electronic devices such as computers, televisions, and smartphones. They are also used as voltage amplifiers in audio equipment, power supplies, and sensors. In addition, transistors are essential components in many electronic circuits and are found in a wide range of consumer and industrial products.

Suggested for: Transistor as switch-op amp voltage amplifier?