![]() That way the large variation in \$\beta\$ is not an issue. We saw in the previous article that BJT beta is both simple and complex: simple, because all you need to do to calculate collector current is multiply base current by beta complex, because there are several betas to choose from. The model parameter SUBS facilitates the modeling of both vertical and lateral geometrics. For BiCMOS devices, use the high current Beta degradation parameters, IKF and IKR, to modify high injection effects. In most circuits, having a \$\beta\$ that is larger is OK. The Importance of the Current Gain of BJTs. The BJT model is used to develop BiCMOS, TTL, and ECL circuits. Because the base voltage is kept close to ground by the NPN BJT (about a diode drop above ground), the current developed by \$R_\text\$. Here, \$V_1\$ is a variable voltage source or else a ramp or sawtooth voltage. ![]() Simulate this circuit – Schematic created using CircuitLab Here's a simplified diagram using an NPN under test: You'll want a method for varying the base current in a "smooth" fashion over some desired range. Enough to be annoying.ĭesigning and using your own system for estimating \$\beta\$ can be fun, though. It's more likely that if you buy two different multimeters they will read somewhat different values for the same BJT. And their value usually isn't particularly accurate, either. There is no international standard used by multimeter manufacturers. Effective beta changes based on current - at low currents, base-emitter leakage and non-ideal B-E junction effects reduce the effective beta at high currents, other non-ideal effects (high level injection) also reduce effective beta. I really cannot figure out what I'm doing wrong here.Your question pretty much sums up the problem, doesn't it? You write, BF is the parameter used to describe the maximum beta of the device. I'm really confused.įor those old designs, the ones I initially thought should work, I duplicated the same circuit with my old 2N3904s, and the values were right on the money. What gives guys? I thought maybe the spec for the transistor drifts a lot when it's outside of the measured conditions (5V 2mA Ic), but I used a 5V supply as well and the results are the same. This time, the values agree! The emitter voltage is about 940 mV, and the base voltage is about 1.54 V. ![]() I worked out values of 18k for R1 and 2.8k for R2 (bias resistance of about 2.42k). So, with that, the expected resistance looking into the base is about 20k. Eventually, I tried to work out the beta value backwards, and it looks like the transistor has a real beta value of about 43-44, not even close to the claimed 420-800. I tried to reduce the total base resistance to reduce the loading effect, but that made it worse. I've built a few more circuits with different values. Needless to say, I'm definitely not getting 2mA from this source. I expected to be at least somewhat close to a volt, but this is less than half. The emitter voltage worked out to be 447 mV. The divider above should work out to a Thevenin Equivalent of about 28.7k, pretty dang close.Īfter the quick sketch, I built the thing and to my surprise my design did not work out at all. This means I'll have to aim for a base resistance of about 28k and a base voltage of about 1.5-1.6 To bias the transistor, I multiplied the emitter resistance by beta (assumed 600) and divided by 10 as a rule of thumb. So, if the base current, Ib, going into a transistor is 30A, the emitter current will be. So if a transistor has a of 100, the base current that goes into it is amplified by 100. It is the factor by which current is amplified in the circuit. I want to put the emitter at about 1V, so at 1V / 2 mA gives 500 Ohms, I went with 470 Ohms as the closest value I have. (beta) of a transistor is the gain or amplification factor of a transistor. ![]() The actual range is actually like 420 - 800 or something.Īnyway, I was trying to build a current source for a differential pair and wanted to pump out about 2mA of current through the transistor, so I set it up like this for a quick test: Reading through the datasheet, (and on digikey's page), I am led to believe that the DC gain for this transistor is at minimum 420 2mA, 5V]. I bought these transistors on digikey: BC549CTA
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