Transistor basics: Load on collector or emitter?

Go To Last Post
17 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I would be grateful for clarification on how to connect a load up to a transistor. In particular, when should the load be connected to the collector and when to connect the load to the emmiter. I've seen circuits containing both these configurations.

regards,

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You really need to say what you are connecting.

Then you google for schematics of how others connect the load.

Since NPN transistors are probably cheaper than PNP, it is common to connect the NPN Emitter to GND, Collector to load, Base to AVR via a 10k resistor. The other end of the load goes to 5V.

Then the load will be 'on' when the AVR port is '1', and off when port is '0'.

It is wise to have a 10k 'pull-down' resistor between Base and GND. This means that your nuclear bomb is only detonated when your port is both o/p and '1'.

David.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Is the transistor working in analog or digital mode?
Connecting a load between the emitter and ground means the maximum voltage out will be Vcc-0.7V, not really useful for digital.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

When the load is connected between the emiter and signal ground that configuration is know as emiter follower.

Emiter follower configuration has slightly less than unity voltage gain. It does however have a very low output impedance and a high input impedance.
Often used as a buffer stage. buffer stage should ideally have very high input impedance and a good broadband output impedance.

Alternatively when the load is in the collector the topology is refered to as common emiter. Input impedance tends to be lower. Output impedance tends to be high.
It can have a decent voltage gain at the expense of stage bandwidth.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Among the other things that have been told,in the common colector or emiter follower circuit the input signal and output signal have zero phase shift.
The common emitter circuit causes 180 phase shift between input and output signal.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Adding to what has already been said, there are two aspects to a transistor design.
-input/output configuration,DC or AC amplifier and frequency response
-the biasing of the transistor to get it to work in the required mode.

So this is where the answers to the OP's question are somewhat fragmented, because it depends on what the OP wants to do. It is a can of worms that is not easily explained in a few lines.
text books have been written about it. I recommend The Art of Electronic Design by Horowitz & Hill

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thanks everyone.

I'm sorry, but I'm still a bit lost really.

I'm not referring to any specific application that I am currently building. My question is one for my own general understanding on how to connect the load to the transistor (via the emitter or collector) so I can take these reasons into account in my designs. I have been working with trial and error and dependent on google really.

Someone also mentioned here that a 10K resistor between base and AVR for general applications. In my experience I have always had to use a lower resistor, such as 220ohms on the base saturate the transistor sufficiently to drive things such as small 5V motors and relays etc.

Regards,

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:
Someone also mentioned here that a 10K resistor between base and AVR for general applications.

Incorrect. Someone mentioned:
Quote:
It is wise to have a 10k 'pull-down' resistor between Base and GND.

Quote:
...so I can take these reasons into account in my designs.

It is not as simplistic as what you think!
Quote:
In my designs.....I have been working with trial and error and dependent on google really.

Not a really good idea. You might get somethings to sort of work, some of the time but they may not be stable, they may not be "fail safe", get hot, smoke, start fires, burn your house down, cause loss of life...etc.
There are somethings you need to understand if you propose to do regular design. That is why engineers are such highly paid professionals! :lol:

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Rhoaste,

Are you truying to interface real wolrd to AVR digital outputs?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:
That is why engineers are such highly paid professionals!
WHAT??? :roll: no one ever told me that.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

rhoaste,

It might be worth getting a copy of LTSpice and looking at some of its example circuits.

There is info on the website sticky area on this software I believe.

IMHO, Common emitter circuits are a good place to start.

oddbudman

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

rhoaste wrote:
Thanks everyone.

I'm sorry, but I'm still a bit lost really.

I'm not referring to any specific application that I am currently building. My question is one for my own general understanding on how to connect the load to the transistor (via the emitter or collector) so I can take these reasons into account in my designs. I have been working with trial and error and dependent on google really.

Someone also mentioned here that a 10K resistor between base and AVR for general applications. In my experience I have always had to use a lower resistor, such as 220ohms on the base saturate the transistor sufficiently to drive things such as small 5V motors and relays etc.

Regards,

What, these things are not in electronics books these days?

Well it depends are you trying to drive digital things on/off like leds, motors and relays or drive analog things like speakers etc. For digital control you almost always put the load on collector and ground the emitter if you have a NPN transistor.

Transistors can be thought as current amplifiers. As a rule of thumb you can approximate a regular small-signal transistor to have a current gain of about 100. So a 100mA motor on collector needs 1mA of base current. 10kohm resistor cannot provide that. Normally 4k7 would be good there but people may put some safety factor there and just put a 1k base resistor. The collector-emitter voltage is about 0.2V when transistor is fully on, so in a 5V system your load gets about 4.8V at high currents.

Another nice feature of the transistor is that the base-emitter voltage is about 0.7V, so if you put the load on emitter and drive the base with AVR, the load can never have more than 4.3V over it in 5V system, i.e. the emitter voltage follows the base voltage. Good for buffering say audio signals generated from AVR.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

rhoaste wrote:
Someone also mentioned here that a 10K resistor between base and AVR for general applications. In my experience I have always had to use a lower resistor, such as 220ohms on the base saturate the transistor sufficiently to drive things such as small 5V motors and relays etc.

This is why you were asked what you want to switch on and off.
1. 2mA LED
2. 20mA LED
3. 50mA LED backlight
4. 1000mA high-power LED
5. relays, motors, nuclear detonators...

(1) is simpler direct from the AVR.
(2),(3) are easily done with a small NPN transistor with the load between 5V and collector.
(5) can be done the same for small currents. You need a protective diode across the relay coil. (or RC)
(4) needs careful choice of transistor. i.e. you need to calculate the base current, and check data sheets.

Your simple general purpose 2..100mA type of transistor switch will be fine with a 10k base resistor. (500uA base current and a hFE of 400)

The reason for the 10k base-emitter resistor is to ensure that your transistor does not turn on when the PORT is configured as an i/p. (i.e. at RESET)
Any CMOS pin is very sensitive to noise when it is an i/p. Even a tiny 2uA leakage current could light your 2mA LED (2uA x hFE = 800uA is visible).

So yes, a lower value pull-down might be safer with nuclear devices. But I would feel safer with a nuclear designer that has done her schoolwork.

I think that you are referring to a 220R base resistor from the AVR pin. This will supply 23mA of base current to your power transistor. Power transistors tend to have hFE of 20..100 rather than the small transistors with high hFE of 200..600

You can do simple rule-of-thumb calculations. My thumbs are a different size to LDEVRIES's thumb. However a 2mA device is very different to a 10000mA device.

David.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

LDEVRIES wrote:
Quote:
Someone also mentioned here that a 10K resistor between base and AVR for general applications.

Incorrect. Someone mentioned:
Quote:
It is wise to have a 10k 'pull-down' resistor between Base and GND.

Incorrect? I dont think so. Someone did mention exactly that...

david.prentice wrote:
Since NPN transistors are probably cheaper than PNP, it is common to connect the NPN Emitter to GND, Collector to load, Base to AVR via a 10k resistor.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I'll add to the noise here and say you won't go too far wrong by using the common emitter connection, with the emitter to GND and the collector to load to V+. One advantage is that the V+ can be different than the AVR Vcc, so e.g. you can drive 12 or 24V loads with a 5V micro.

And note that you can also get MOSFET transistors that don't require any base resistors, and you can get "digital" transistors that have the base resistor built in.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

This is very simplistic and not ALWAYS true but, if you are looking for -

Voltage gain: Collector.
Current gain: Emitter.

Greg

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

rhoaste wrote:
LDEVRIES wrote:
Quote:
Someone also mentioned here that a 10K resistor between base and AVR for general applications.

Incorrect. Someone mentioned:
Quote:
It is wise to have a 10k 'pull-down' resistor between Base and GND.

Incorrect? I dont think so. Someone did mention exactly that...

david.prentice wrote:
Since NPN transistors are probably cheaper than PNP, it is common to connect the NPN Emitter to GND, Collector to load, Base to AVR via a 10k resistor.

Both are correct. The 10k base resistor to AVR IO pin is a bit on the high side but good for driving light loads, say around up to 50mA on collector. The 10k base resistor to GND keeps the transistor off before AVR gets out of reset and configures the IO pin to output.