# BJT conceptual issue or is my circuit bad?

Working on a lab that is the IV characteristics of BJTs. I constructed the circuit as depicted in the lab, I think. The lab wants me to measure the V(collector-emitter) and I(collector) for values of V(Collector) from 0-10v. Do this for values of I(base) of 0,50uA,100uA,150uA,200uA. Now I did this simulation in Pspice before doing the real circuit, and received the following. a plot of Ic as a function of Vce, where each curve represents increasing values of Ib. This graph makes sense. My issue is when I construct the circuit and measure the values, despite changing Ib, there is no change in Ic. The graph of Ic Vs Vce should shift upwards as depicted by my Pspice simulation.

Now, to get the values of Ib, I set the base voltage (Vb) to 1 volt and said that due to this relationship

Ib = Vb - Vbe / Rb that if I keep Vb constant and know what Ib I want then all I have to do is change Rb. So for an Ib of 50uA I calculated an Rb of 6000, for Ib of 100uA, Rb = 3000 and so on.

The issue is that if I have Vb = 1, and Vc = 1 when changing Rb from 6000 to 3000, there is no change seen in Ic.

I really dont know what I am doing wrong. Ive been sitting in the lab for a while. It is springbreak, no one is here. I don't know if I am building the circuit wrong or I have a bad conceptual understanding.

My exact readings are

with a 6k resistor and with a 3k resistor for Rb

PSU(Vcc) - 1v and 0.010A CV

Vce - 1.00655Vdc

Ic - 0.0965uAdc

PSU(Vb) - 01.00V and 0.0A CV

Using a 2N3904 NPN transistor

The lab circuit

NOTE: Ignore the 27k ohm resistor in this picture. This lab is for people with Elvis boards, we don't use them. We also don't have a power supply that can supply microamps. My circuit

(sorry the picture is huge from my phone)

http://imgur.com/a/r87Dw

• is R2 really 500 ohms? – FrancoVS Mar 14 '17 at 3:02
• We need the two sets of measurements with Rb = 3k and 6k. You only have one measurement there. – Kevin White Mar 14 '17 at 3:09
• That's the thing Kevin, the measurements do not change when I swap out the 6k resistor for the 3k. Yes, R2 is really 500 ohms. – MathWannaBe456 Mar 14 '17 at 3:24
• verify Vbe for input sweep – Sunnyskyguy EE75 Mar 14 '17 at 4:24

## 1 Answer

I tried a 3rd BJT (of the same type) on an off chance I got 2 duds. It appears so. Started working immediately. The measured base current was damn near what I calculated.