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Author Topic: transistor current gain vs voltage gain.  (Read 857 times)
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for example...

method 1

by triggering base, we switch on a larger voltage by sinking? eg + side of the led would go to the 5v+ rail... useful as, to power devices with a higher voltage rating...

but..

method 2

current gain? 1ma 100ma to power an led? which is better?  and how do you decide which method is the best ?




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for example...

method 1

by triggering base, we switch on a larger voltage by sinking? eg + side of the led would go to the 5v+ rail... useful as, to power devices with a higher voltage rating...

but..

method 2

current gain? 1ma 100ma to power an led? which is better?  and how do you decide which method is the best ?






I think you are over thinking the topic and giving the transistor more talent then it really has. The name transistor (as applied to standard NPN & PNP devices not mosfets) is derived from 'transfer of resistance'. A transistor is basicly a current operated device where the emitter/base current has a high leverage effect (current gain) on what emitter/collector current is allowed to flow. Only in context with the external circuitry and components wired to the transistor is there a net voltage and/or current gain for the circuit.

 I know this may not be answering your question directly, but what I'm getting at as you have to show at least two different circuit configuration drawings to make your question clear and help answer if one is more proper, more efficient, or simpler for the task at hand.

Lefty
« Last Edit: January 08, 2013, 10:02:31 am by retrolefty » Logged

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Method 1 sounds like using NPN to sink current from cathode to turn LED on.
NPN controllable from Arduino 0/5V output, Voltage source to LED Anode may be > 5V (ex. 12V powering 3 LEDs in series).

Method 2 sounds like using PNP to source current the LED Anode, with cathode to Gnd.
Can work from 5V source with Adruino controlling the base.
Cannot work from 12V source, needs another transistor to pull the base low to turn on, and pullup resistor to 12V bring the base high to turn it off.

If using LEDs, don't forget current limit resistors.

NPN may be replaced with Logic Level N-channel  MOSFET.
PNP may be replaced with Logic Level P-channel MOSFET with 5V source, and Standard Level P-channel MOSFET with >= 10V source.
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Transistors do not have voltage gain as such, only current gain. They are current operated devices.
Once you add one or more resistors in a complete circuit, you turn a voltage into a current. However, the transistor still works on current.
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There are two types of transistors, which have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing between the base and the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain.

I'll leave it But Bipolar transistor are current controlled
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I'll leave it But Bipolar transistor are current controlled
I thought that is what he just said?
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Transistors are inherently current gain devices, but that's not usually how we use 'em..    Most signals & sources we use are voltage-based*, so ttansistors are typically used in circuits that amplify or shift voltage.  Or, they are used as a switch to boost the current capability, rather than amplifying (multiplying) the current by some constant gain-factor.

The current gain (hfe) specs are usually very loose, and it varies with temperature, so it's usually a bad idea to use a transistor alone to control/regulate current.    If you want to use a transistor as a current source, there is usually some kind of feedback to measure and adjust the current, keeping it constant.



* For example, as long as we have normal operating conditions, the voltage-output from a microphone or audio amplifier doesn't change with the load resistance.    If we have an amplifier with a voltage gain of 100, the voltage gain is independent of the load resistance.   But, the current (and therefore the current gain) depends on the load resistance.   (But, we never think about the current gain.)  

Or if we have a 12V 1 Amp power supply, it's supposed to put-out 12V no matter what load we attach, as long as we don't exceed the 1A limit.   (How well it holds the 12V constant depends on how well the power supply is regulated.
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Grumpy_Mike You had not posted and when I did your post showed up so I left the quote. it just added to your post.
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i was overly thinking it ....

what my question should have asked....

when to sink/source ? how to know which to use? (in relation to the fact the main power supply would be 5v)


use the transistor source
or
use the transistor as a sink

both 5v supplies

or does it really not matter? cheers
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i was overly thinking it ....

what my question should have asked....

when to sink/source ? how to know which to use? (in relation to the fact the main power supply would be 5v)


use the transistor source
or
use the transistor as a sink

both 5v supplies

or does it really not matter? cheers


Sometimes either way can be made to work, but perhaps one method uses fewer components or cheaper commonly available parts. Designing electronic circuits is not rule based but rather a creative process that can have different design decisions made by different designers. Engineers may have their own personal 'favorite circuits' to use for common needs that might not be the choice made by other engineers. Learn the fundamentals well and you too will be able to learn to make various trade off decisions in your own designs.

Lefty
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N-channel MOSFET and NPN transistors seem to be more readily available and less expensive from what I have seen in searches at digikey.com.
They are both drivable as current sinks from Arduino directly, while anything but a 5V source requires a pre-driver for PNP/P-channel to be able to turn them off.

* Transistors - BJT.pdf (172.78 KB - downloaded 23 times.)
* Transistors - MOSFET.pdf (201.85 KB - downloaded 22 times.)
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