Using a transistor as a (relatively) high speed switch

In summary, you can use a transistor to switch a current for a short pulse length, but you may want to look into a voltage controlled resistor if you want a repeatable switch.
  • #1
VinnyCee
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0
So I'd like to pulse a current to a load from a transistor. I can use a µ-controller to trigger a transistor. The µ only has a minimum pulse length of about 100ns or greater. But I would like to have the current flow through the transistor for a much shorter pulse length than 100ns. Maybe a pulse length of 5ns or even 10ns would do.

1) What is the best way to do this type of fast switching? Should I have the current always flowing through the transistor and then turn it off for the short pulse length, or have the current off normally and have the transistor control when it flows (for 5ns or 10ns)?

2) What options do I have regarding specific model numbers?
 
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  • #2
VinnyCee said:
So I'd like to pulse a current to a load from a transistor. I can use a µ-controller to trigger a transistor. The µ only has a minimum pulse length of about 100ns or greater. But I would like to have the current flow through the transistor for a much shorter pulse length than 100ns. Maybe a pulse length of 5ns or even 10ns would do.

1) What is the best way to do this type of fast switching? Should I have the current always flowing through the transistor and then turn it off for the short pulse length, or have the current off normally and have the transistor control when it flows (for 5ns or 10ns)?

2) What options do I have regarding specific model numbers?

What is the application? Getting that kind of large-signal switching speed out of discrete transistors is non-trivial. Can you just use a fast logic gate instead to source/sink the current?
 
  • #3
I need a low cost method of switching, and the transistor is the lowest cost device I think. A logic gate would be a lot more expensive right?

I am charging a capacitor and then using the switch to dissipate the charge from the capacitor into another, unknown, resistance and capacitance. And measuring the voltage before and after.

The 5ns pulse would be ideal, but a 100ns or less pulse would also work I suppose.
 
  • #4
VinnyCee said:
I need a low cost method of switching, and the transistor is the lowest cost device I think. A logic gate would be a lot more expensive right?

I am charging a capacitor and then using the switch to dissipate the charge from the capacitor into another, unknown, resistance and capacitance. And measuring the voltage before and after.

The 5ns pulse would be ideal, but a 100ns or less pulse would also work I suppose.

It sounds like you need an analog switch function. Why so fast?
 
  • #5
Clarification:

I am really looking for a part, in production, that can respond to the input pulse consistently. I've seen some datasheets (for example, this one) list the turn on time as 9ns typical to 12ns maximum and a turn off time as 13ns typical to 18ns maximum. I would want the difference between these typical and maximum values to be as little as possible.

Also, what is the "storage time" listed in that datasheet?

The µ-controller that will be sending the control pulse to the switch (i.e. ~ this transistor) will have a 100ns minimum pulse length currently, but that may change. Repeatable because there will be 8 of these in parallel, each doing multiple measurements every 100µs or less.

I just need a low cost option for repeatable switching using a transistor instead of an actual, packaged switch.


I think what I'm seeking is actually a Voltage Controlled Resistor. This is commonly made with a JFET, right?
 
Last edited:

What is a transistor and how does it work as a high speed switch?

A transistor is a semiconductor device that can amplify or switch electronic signals. It consists of a base, emitter, and collector, and works by controlling the flow of current between the base and collector using a small current at the base.

Why would someone want to use a transistor as a high speed switch?

A transistor can switch electronic signals at a much faster rate than traditional mechanical switches, making it a valuable tool for high speed applications such as in computers, telecommunications, and other electronic devices.

What factors affect the speed of a transistor used as a switch?

The speed of a transistor as a switch can be affected by various factors such as the material used to make the transistor, its size, and the voltage applied to it. Additionally, the circuit design and other components in the circuit can also impact the speed of the transistor switch.

What are the advantages of using a transistor as a high speed switch?

Some advantages of using a transistor as a high speed switch include its small size, low power consumption, and ability to switch signals at a fast rate. It also has a longer lifespan compared to mechanical switches and can be easily integrated into electronic circuits.

What are some common applications of using a transistor as a high speed switch?

Transistors are commonly used as high speed switches in various electronic devices such as computers, televisions, and smartphones. They are also used in communication systems, power supplies, and other industrial applications where fast switching is required.

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