*voltage*. BJTs also exhibit transconductance, which gives us another way to evaluate gain. Remember we talked about how hfe has pretty large manufacturing tolerances and varies from device to device? Transconductance is just the opposite. It's extremely predictable and is the same for every part number, silicon and germanium! That's because transconductance in BJTs is a fundamental property of solid-state physics. Transconductance is represented by the symbol

**gm**. gm is easily calculated; it is collector current divided by the Thermal Voltage (vt), which is about 26mV at room temp: gm = ic / vt. For example, a transistor with a collector current of 520uA would have the transconductance 520uA / 26mV = 20mS (milliSiemens). Conductance is measured in units called Siemens (go ahead, get your snickering out of the way). Siemens is the reciprocal of Ohms. A 1K resistor has a conductance of 1/ 1K = 1mS. To calculate the voltage gain of a transistor stage, we multiply the transconductance by the load resistance. Suppose the transistor in the previous example has a 15K load resistor. Its voltage gain would be 20mS * 15K = 300. Ok, so it's not quite that simple because BJTs have output impedance which is effectively in parallel with the load resistance. We can get a pretty good estimate of a transistor's output impedance by dividing 50V by the collector current. It's just a rule of thumb, but it's plenty accurate for what we're doing. The transistor in the previous example has a collector current = 520uA, so the output impedance is approx. 50V / 520uA = 96K. We put that in parallel with the 15K load resistor and get 13K, so the voltage gain is actually 260. Notice that hfe plays no part in calculating transconductance or voltage gain. A germanium transistor with hfe = 50 has the same transconductance as a silicon transistor with hfe = 800, as long as their collector currents are the same.

Two more quick thoughts related to transconductance and we'll call it a day.

If we know hfe & gm, we can calculated the input impedance (Zin) of a transistor. Zin = hfe / gm. Let's say the transistor in the previous examples has hfe = 200. Then Zin = 200 / 20mS = 10KΩ.

The output impedance of an emitter follower is low, but how low? That's easy to calculate. The output impedance (Zout) of an emitter follower is Zout = 1 / gm. Using that same transistor as an emitter follower: Zout = 1 / 20mS = 50Ω.

Next time: What's All This Impedance Stuff About?