# what's the purpose of this transistor setup (answer: simulate an inductance)

I encounter the following block in three regions of an Altec Lansing APT3 2.1 speaker system.

simulate this circuit – Schematic created using CircuitLab

Given that VH = 9V D.C., I only understand this as an elaborate voltage divider. What would be the purpose of that construction wrt VD ?

--- Edit ---

Given Olin Lathrop's answer, here's the context in which this circuit is used.

simulate this circuit

• why don't you scan an image of the real circuit instead of drawing something that doesn't seem to make sense. Oct 26, 2013 at 17:09
• The way this circuit is drawn obfuscates what is going on. Try to draw the circuit logically with power at top, ground at bottom, and signals flowing left to right. It is also necessary to show which point is a input or output or otherwise connected to externally to say what the function of the circuit is. Oct 26, 2013 at 17:34
• Sorry! It's the first time I'm doing this. I hope the schematic is clearer now. I however doubt that showing you the picture of the PCB would help in that respect, but I can do it if you think it's useful. Oct 27, 2013 at 13:35
• Do you actually have a schematic of the real circuit or is this your best guess at what the circuit is? VD must connect to something? Oct 27, 2013 at 15:16
• it's my best guess. VD is connected to the rest of the circuit which I believe we can treat as having a large input impedance. Oct 27, 2013 at 15:32

## 1 Answer

Your redrawn circuit is better, but still a little obfuscated. Here is the same circuit drawn more intuitively:

Parts of this make sense, but I don't know what the purpose of connecting anything to VD would be. R1, R2, and R4 are clearly there to bias the transistor, and R3 looks like a typical emitter resistor, but R5 and C1 don't make a lot of sense if VD is the only external connection to this circuit other than power and ground.

This circuit could possibly be producing the bias voltage VD for use elsewhere. The feedback of AC signal on VD to the base would have the effect of making the impedance relatively high against short term variations (AC), while keeping the DC bias level and impedance to DC constant. Put another way, this could be a quick and dirty electronic version of a inductor tied to a bias supply.

• Would this be a en.wikipedia.org/wiki/Bias_tee ? (see update in question) Oct 27, 2013 at 16:16
• @yannick: Yes, it looks like a electronic version of that. Inductors are big and klunky and expensive. For some applications, it is better to emulate them with active electronics, as this circuit appears to be doing. Oct 27, 2013 at 16:20