Basic Transistor Question

[Phrazemaster]

Apologies if this belongs in "The Lab" or somewhere else; if so mod pls move.

I have a simple question regarding operation of an amplifier using a Transistor - pic attached. While I (think) I get the basic principle of the transistor, what is the purpose of the cap and additional voltage divider in red? I understand of course the need for the Input signal, but not the associated cap and voltage divider.

I'm getting ready for an 1176 build so I figured I prolly oughtta understand the basic principles behind it.

BTW here's an awesome tutorial on transistors on YouTube; the guy explaining this stuff is hilarious and so well-spoken. In the vid he does give a passing explanation to what I'm asking, but I'm looking to really understand this better.

https://www.youtube.com/watch?v=4QkRI1Ue208

And this more introductory one:

https://www.youtube.com/watch?v=CkX8SkTgB0g

Thanks in advance for kind souls willing to share what's up.

Mike

 

[john12ax7]

The resistors set the dc bias of the transistor, the cap blocks that dc. Essentially you are setting the nominal quiescent current of the transistor.

 

[Phrazemaster]

The resistors set the dc bias of the transistor, the cap blocks that dc. Essentially you are setting the nominal quiescent current of the transistor.

Thank-you John, I understand about the resistors setting the DC bias - makes sense; it's a voltage divider.

I'm not clear on how the cap blocks the DC. The cap appears before the voltage divider - so how would it block DC going to the base?

Sorry if it's obvious. TIA.

Mike

 

[john12ax7]

You want the dc to get to the base, it doesn't block that. It prevents the dc from going to the previous stage. It would also  prevent any dc from a previous stage messing up the base bias.

 

[Phrazemaster]

OK that makes sense - much appreciated! I just couldn't understand from the videos how that cap was supposed to protect the transistor from DC, when DC is what is biasing it!

Thanks again man!

Mike

 

[JohnRoberts]

OK that makes sense - much appreciated! I just couldn't understand from the videos how that cap was supposed to protect the transistor from DC, when DC is what is biasing it!

Thanks again man!

Mike

The cap isolates the input from the DC. The AC input will likely be referenced to 0V DC.

JR 

 

[Phrazemaster]

The cap isolates the input from the DC. The AC input will likely be referenced to 0V DC.

JR

The Vcc that goes through voltage divider R2 and R3 carry DC voltage, correct? So DC is what biases the base through that voltage divider, is that right?

Are you saying that cap blocks any DC that may be present in the input signal from getting into the base as well, which would distort the bias point?

Sorry for being obtuse; I really want to understand this.

Also, although not shown in this schematic snippet, I have seen the output with a cap on it too - presumably to strip out any remaining DC that came from biasing the transistor?

 

[bluebird]

Are you saying that cap blocks any DC that may be present in the input signal from getting into the base as well, which would distort the bias point?

Yes the bias voltage is to get the transistor turned on and to get a small amount of current flowing. The AC signal coming in from the other side of the cap offsets bias point up and down from where it is set with the bias resistors.  Causing the current in the transistor (between emitter and collector) to flow according to the fluctuating offset the incoming AC signal is making.

For instance, If you had a guitar pick up on the other side of the cap, referenced to ground, the small fluctuations in AC would wiggle the transistor bias point around where it is set. If there was no cap, DC current would flow through the guitar pick up to ground and mess up the bias point. The cap blocks the DC current but acknowledges AC changes in current.

 

[Phrazemaster]

Yes the bias voltage is to get the transistor turned on and to get a small amount of current flowing. The AC signal coming in from the other side of the cap offsets bias point up and down from where it is set with the bias resistors.  Causing the current in the transistor (between emitter and collector) to flow according to the fluctuating offset the incoming AC signal is making.

For instance, If you had a guitar pick up on the other side of the cap, referenced to ground, the small fluctuations in AC would wiggle the transistor bias point around where it is set. If there was no cap, DC current would flow through the guitar pick up to ground and mess up the bias point. The cap blocks the DC current but acknowledges AC changes in current.

Very well stated, and I understood that!

Thank-you Bluebird!

 

[warpie]

And how do you calculate the value of the cap? 

 

[Phrazemaster]

Me personally I wouldn't know how to calculate the value of anything here yet - I'm just trying to understand the basic concept first.

But if anyone wants to chime in, eyeballs await...