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Transistor as a switch

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Jul 18, 2015
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I want to design transistor as a switch.Any ideas on how can i begin.Every text that we have been taught uptil now discusses it but is silent about design.

what type?
**broken link removed**

Most BJT's are rated as a switch for Vce(sat)= ?V @ Ic/Ib=10 The above suggests this ratio to be 20, which may result in higher Vce(sat) Some very high gain (hFE=500) types with ultralow Vce(sat) are rated for 50 or 2% base current.

Start with base current, Ib =10% of Ic and learn to choose devices rated for 20 or 50 :1 where needed.

Design? You want the calculations (e.g what would be the base current to drive transistor in full conduction - saturation?) or the schematics?

Simply look at the datasheet of the switching transistor.
Here is a little 2N3904 transistor switching a 50mA current and a big 2N3055 power transistor switching a 10A current:


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    switching transistor.png
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I want to design transistor as a switch.Any ideas on how can i begin.Every text that we have been taught uptil now discusses it but is silent about design.

I do not know what text you used but those I know offer an easy design procedure.
You should know what current you need to switch and under what voltage.
The switching transistor has two states: closed, with no base current. Open, full base current.

Depending on current value you should select a transistor that can handle it. Voltage is also good to know.
Circuit to use can connect a transistor in parallel with the load, or in series. A typical switching transistor when open has a "saturated" voltage across it, so the load cannot reach the full DC voltage.

Some advantages can be used by using switching MOSFET which has a very low "open" resistance. Slightly different design rules.

Switching speed is often required to be specified. This may call for a "fast" transistor.
If a load is inductive, the transistor must be protected by a parallel diode from overshoots.

All above and more can be found in any electronics textbook . I would recommend you to find the ARRL Radio Amateurs' Handbook, any issue is full of good advices.


just below these posts you find a box "similar threads".

Isn´t this a good feature of the forum?


Can you tell me what frequency of input would be fine?I am using 2N3904.


Can you tell me what frequency of input would be fine?I am using 2N3904.
This is not a question of what transistor you use.
It is the qustion what you want to switch. It is your application. We don´t know.


I understand everything except the part where he assumes hfe.What does assuming hfe means?

Generally BJT's Ib requires 10% of collector current for best saturation and fastest speed.

Since linear current gain hFE varies widely and drops rapidly to about 10% of max when saturated, it depends on the device characteristics for Vce(sat) used vs Ic/Ib.

MAx Frequency - Transition 250MHz assumes unity gain emitter follower.

But used as an inverting switch, you depend on Ic/Ib ratio gain for saturation thus bandwidth is reduce by this value and is affected by Vce and drive current. But ideally you can switch 25 MHz give or take depending on load and Vce.

Complementary output stages yield much higher switching rates in both BJT's and FET's. (CMOS)

hfe is AC current gain and hFE is DC current gain only when a transistor is linear with plenty of collector to emitter voltage and is not a saturated switch.
The datasheet for a 2N3904 shows an hFE from 100 to 300 when its collector current is around 10mA and its collector to emitter voltage is 1V or more. A graph shows its hfe is the same for low frequencies then it drops above 1MHz so that at 6MHz it has no current gain.

The datasheet shows that for it to switch DC on and off its base current must be 1/10th its collector current so the gain is assumed to be only 10 times.

Why do you want to use a transistor to switch a frequency on and off? Instead why don't you switch the frequency's detector, generator or amplifier on and off? Frequently a transmission gate (CD4066 or multiple) is used to switch a frequency on and off.

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