Transistor usage question

[GeekDoc 226]

When I wanted a more sensitive IR sensor for my project, I (basically) tied the emitters and collectors of two IR phototransistors together. The bases were effectively tied, since they were both pointed at the same IR source. This brings my question:

 

If you tie all the leads of two NPN transistors together, could you effectively amplify a voltage, even though the transistors are in a "voltage following" configuration? i.e.: collectors tied to 3v, emitters tied to an MSP430 i/o pin, bases tied together. Apply 1.5v to base. Can you get close to 3v at i/o pin?

 

I've been wondering because I've been trying to figure out a minimal-parts solution to gatesphere's PIR sensor problem (only gets 1.7v signal when run off 3v; not enough to trigger MSP430, especially with pull-down resistor). If I had a little more time, I'd try this out myself, but I thought someone might know.

[bluehash 1,581]

I've been wondering because I've been trying to figure out a minimal-parts solution to gatesphere's PIR sensor problem (only gets 1.7v signal when run off 3v; not enough to trigger MSP430, especially with pull-down resistor). If I had a little more time, I'd try this out myself, but I thought someone might know.

I'm guessing here, but if you tie the output of the PIR (1.7V) to the base of an NPN, collector to 3.3V voa a 1K resistor, emitter to GND and the junction of the resistor and collector to the Launchpad input pin, it should resistor as a high.

[gatesphere 45]

As I'm receiving some transistors from Doc (thanks, man!), I'll look into this solution myself and do a blog write up. But before I do, I'd like to have some background in transistors... I have no idea how to use them. Doc, do you (or anyone else) have any good resources to explain how to use them and how they work? I just started my EE odyssey this summer, so I'm sill a bit (read:VERY) green.

[jsolarski 94]

I will post a something later tonight or tomorrow, about transistor use. there are lots of different configurations, like a darlington pair, or a simple amplifier. lol i still have lots of projects that use NPN transistors and have a few odds and ends that should help

[gatesphere 45]

Thanks, that'd be great!

[NJC 17]

If you tie all the leads of two NPN transistors together, could you effectively amplify a voltage, even though the transistors are in a "voltage following" configuration?

 

If you tie the transistors together it would be just as if you had one there. Except maybe for current considerations. Because all pins would be connected they would all share the same voltage. I would not recommend doing that though.

 

Building a simple 1 transistor amplifier is not nearly as simple as using op-amps. To solve gate spheres problem you could use either a comparator or an op-amp. If you were intent to use a transistor, you still can. I assume you are using BJT's since you said "base". MOSFET's and BJT's function on slightly different principles and as such use different equations.

 

I have no idea how to use them. Doc, do you (or anyone else) have any good resources to explain how to use them and how they work?

 

http://users.ece.gatech.edu/mleach/ece3050/notes/bjt/bjtbias.pdf

 

It's not the best link, but I found it quickly and the equations are presented fairly coherently. Beta (and thus alpha) are properties of the transistor. Most times beta is shown as hfe in the data sheets of the transistors. Basically it all comes down to how well you can do circuit analysis. To be honest, I wouldn't bother spending too much time delving into the details of transistors. As a hobbyist it almost always makes more sense to use op-amps; and when you need to use a transistor you can design the circuit using any SPICE software and a few example schematics from the internet. In Electrical Engineering you take a years worth of transistor design classes quite far into your degree. If you are really interested in learning I can recommend a text book which does a great job on going over transistors. Microelectronic Circuits - by Sedra and Smith. Otherwise start with the basic equations of steady state analysis and then work your way into the small signal models.

 

If want to learn more though, I'll do my best to help when you get stuck.

 

NJC

[jsolarski 94]

NJC-To be honest, I wouldn't bother spending too much time delving into the details of transistors. As a hobbyist it almost always makes more sense to use op-amps

thats not always true, i have used both transistors and op-amps, and would prefer to use Transistors, but for something as simple as just boosting the analog signal from a sensor is fairly easy to do with a transistor and a resistor. Op-amps you will have to add a few more components, but i shy away from op-amps(maybe from the lack of experience).

I use transistors to boost LF signals(darlington set) or make a simple multiplexing circuit, or class a or class b amp and thats just using generic BJT transistors.

 

also as i said before i will put a small write up about transistor use and some sample circuits, nothing fancy just a few equations to calculate power consumption and a few other critical values.

 

I really think it comes down to picking the right part for the job.

[gatesphere 45]

Thanks for the information, NJC and jsolarski.

 

I will keep in mind that every part has its place, but I am the type of person who would rather have a bit of knowledge about something, even if I'm not going to use it. So, therefore, I will pursue both transistor and op-amp usage.

 

Thanks for the info though

[NJC 17]

I really think it comes down to picking the right part for the job.

 

Agreed. There is always a best part for the job. I guess I was over-complicating things due to the nature of my own projects. When designing active filters and multi-stage amplifiers it's better to use an op-amp. For one, you have a known frequency response when using op-amps, and second, the input and output impedance of each stage is much easier to find. But in circuits where you need to drive something with a small signal, transistors are the best.

 

Op-amps you will have to add a few more components,

 

That is not always true. As you already mentioned it very much depends on the goal of the project.

 

I wholly agree with you though on the use of standard transistor configurations (ex. common emitter - http://en.wikipedia.org/wiki/Common_emitter) for use in hobby electronics. I should have been more clear about what I meant; I do not think one needs to muddle around in the equations as a hobbyist, unless there is desire to.

 

Looking forward to seeing your post. :-)

 

NJC

[GeekDoc 226]

I've unleashed an information flood! (That's a good thing.)

 

Thank you to all the smart people for coming to our aid!

 

-Doc

[jsolarski 94]

lol almost done with the right up, just adding the circuit diagrams which will take me till later today.

 

 

look for it tonight!!

[gatesphere 45]

Nice

[jsolarski 94]

sry had to put it on hold for a few days, had to get this oscilloscope from a friend.......

[gatesphere 45]

It's fine, take your time! Brilliance doesn't come overnight.

[jsolarski 94]

lol my gimp skills payed off, finally i have a few examples to post with the transistor write up

later today ---before 11am CST