Chopper Transistor: Significance & Pulse Generation Explained

[TheAnalogKid83]

Can anyone explain to me what is the significance of a chopper transistor as opposed to a normal BJT? I read online that its used for square wave pulse generation, but I don't really understand what the mechanism is.

Thanks

 

[MATLABdude]

Can anyone explain to me what is the significance of a chopper transistor as opposed to a normal BJT? I read online that its used for square wave pulse generation, but I don't really understand what the mechanism is.

Thanks

A BJT, MOSFET, JFET, etc. is a device.

A chopper transistor is an application of the above. If it's the chopper transistor I'm thinking of, it just means a transistor used to generate a square wave from a DC supply. The mechanism for doing this is some kind of back-end circuitry (there's lots of ways of doing this, with a 555 timer, for instance) which controls the transistor, turning it on and off as necessary to generate the square wave.

 

[TheAnalogKid83]

A BJT, MOSFET, JFET, etc. is a device.

A chopper transistor is an application of the above. If it's the chopper transistor I'm thinking of, it just means a transistor used to generate a square wave from a DC supply. The mechanism for doing this is some kind of back-end circuitry (there's lots of ways of doing this, with a 555 timer, for instance) which controls the transistor, turning it on and off as necessary to generate the square wave.

Ok, so what distinguishes it as a transistor to be used for chopping applications? As in, why are there parts sold specifically labeled as chopper transistors? What spec or mechanism makes it characterized for this application? Or else you should be able to use any general purpose transistor that meets your applications requirements. But these are specifically sold as chopper transistors.


Example: http://www.centralsemi.com/PDFs/products/CMPT404A_R1.PDF
"designed for chopper
applications."

 

[skeptic2]

A chopper transistor doesn't need to be highly linear for low distortion. Instead the most important parameters are probably the Vce(sat), Ic(max), Pd(max), rise time (tr) and fall time (tf).

 

[MATLABdude]

Probably just properties that are good in chopper applications. You probably *can* use regulator transistors (provided they meet your minimum requirements). Off the top of my head, I'd guess fast transition times (less slew) and high breakdown voltage (to handle inductor transients--since choppers are frequently used to drive motors or in switch mode power supplies, which would rely on the transient response of an inductor to generate high or negative voltages).

EDIT: Beaten by the skeptic

 

[TheAnalogKid83]

Ah thank you very much. So its a switching transistor that can handle spikes that you might normally use a snubber diode for protecting. And you aren't going to try to use it as a small signal amplifier in the active region, so the linear characteristics are not designed for with priority. A high voltage switching BJT that isn't necessarily a high power device.

So this would make sense, as in my application it is in use with an antenna device that may create inductor voltage spikes.

 

[berkeman]

So this would make sense, as in my application it is in use with an antenna device that may create inductor voltage spikes.

Um, are you saying that you want to drive an antenna with a square wave? Why?

 

[TheAnalogKid83]

Um, are you saying that you want to drive an antenna with a square wave? Why?

Its being used as part of a switch to power an active antenna. So basically this chopper will turn on and off the DC voltage path to the antenna dependent on a few conditions (open antenna or short antenna fault detection, enable/disable antenna).

 

[berkeman]

By "active antenna", do you mean a receive-only antenna with a preamp, or do you mean an active array antenna that is electronically steerable?

 

[TheAnalogKid83]

By "active antenna", do you mean a receive-only antenna with a preamp, or do you mean an active array antenna that is electronically steerable?

Receive only, low-noise amplifer antenna with a fixed frequency range

 

[skeptic2]

You probably *can* use regulator transistors (provided they meet your minimum requirements).

Shouldn't that be, "(provided they meet your maximum requirements)"?

 

[MATLABdude]

Shouldn't that be, "(provided they meet your maximum requirements)"?

Semantics, semantics... For as often as it's employed here (guilty as charged!), a hand-wavy smilie would sure come in handy!

 

[TheAnalogKid83]

i'm a nerd