# Help needed to stop thinking in circles

#### RooftopDuvet

Joined Aug 23, 2009
4
Hello all

I'm just getting into electronics, and i'm looking for efficient ways of viewing circuits to best see their purpose.

I'm having trouble finding detailed explanations of how collector feedback for BJTs actually work. There are lots of explanations for this kind of set up, where the feedback current IS the base current.

http://www.globalspec.com/RefArticleImages/EC37F9299171572362E77C38033C0B25_89.jpg

But I cant find any detailed explanations of the more practical circuit where an external voltage is also applied to the base through an impedance.

Are there any good ways of viewing the circuit to get a better feel for what kind of voltage is at the collector, and what kind of impedance the external base source feels, without having to perform a mathematical analysis. Any rule of thumb formulas would be very useful. My head just goes around in circles with the feedback every time I think about it.

Also, this is my first post, so hello to you all. I hope to contribute once I know a bit more worth contributing

Matt

#### Wendy

Joined Mar 24, 2008
22,155
That is a classic biasing scheme that attempts to control thermal runaway with negitive feedback. A common emitter transistor inverts the signal by 180°, the voltage on the collector goes down as the transistor conducts, reducing base bias current.

Not the best way to do it, but it is in the text books.

#### t_n_k

Joined Mar 6, 2009
5,455
Another thing you can do is to download a circuit simulation software package which allows you to play with the circuit parameters with no manipulation of equations involved. You could set up your circuit and also add an input source as well as various voltage and current probes to examine the circuit behavior as you change component values. There are previous threads on the site in which folk have indicated their favorite simulation software. The "free" versions usually have restrictions but for your simple circuit these would be more than adequate. There is usually a bit of a learning curve (most will have an inbuilt tutorial to start you off) but the benefits are worth the effort.

#### KL7AJ

Joined Nov 4, 2008
2,225
I find it's most useful to separate the DC and the AC behavior. Thermal feedback and biasing are best viewed as DC parameters, while the desired signal is AC superimposed on that.

Of course, with DC amplifiers, such as used in instrumentation, the borderlands can be a bit blurred. However, it's usually a good estimate.

Eric

#### RooftopDuvet

Joined Aug 23, 2009
4
Hey there, thanks for the replies. Yeh I can see qualitatively how it helps to stop internal changes in the transistor from altering the desired output but its difficult to get a grasp on how much the size of the feedback impedance affects the size of the output as an easy fraction.

I had a play around with numbers last night and came up with the following equation, would I be right to use it?

So ignoring the size of Vbe for the sake of approximation. If Vs is a sinusoidal voltage on top of an ideal d.c. biasing voltage would it be true to say that....

$$\frac{V_s}{R_b}+\frac{V_{cc}-A_I\cdot{I_B}\cdot{R_c}}{Z}=I_B$$

so that

$$I_B=({\frac{V_s}{R_B}+\frac{V_{cc}}{R_c}})\cdot({1/1+\frac{A_IR_c}{Z}})$$

or

$$I_B=({I_{B0}+\frac{V_{cc}}{R_c}})\cdot{\frac{\frac{Z}{A_I}}{\frac{Z}{A_I}+R_c}$$

...where $$A_I$$ is the current gain of the transistor and $$I_{B0}$$ is the base current without any feedback. So that the overall input current is an amount smaller decided by the ratio of impedances felt by the current-emitter circuit. I'm not sure how to view the $$\frac{V_{cc}}{Z}$$ term though. I guess this is just an extra term resulting from the base having access to Vcc through Z. Although shouldn't this be zero if Z is a capacitor for instance?

Also, if this is right would this imply that Z appears to be β smaller than it is?

Thanks for the suggestion of free circuit software, been looking for some of that.

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#### RooftopDuvet

Joined Aug 23, 2009
4
ahhhrgh what am i doing wrong with my tex?

#### rspuzio

Joined Jan 19, 2009
77
ahhhrgh what am i doing wrong with my tex?
You used the wrong type of slash in the brackets ----
exactly the same mistake the first time I posted
TeX here but, don't worry, you can go back and

#### RooftopDuvet

Joined Aug 23, 2009
4
You used the wrong type of slash in the brackets ----