# Why isn't the emitter current, $I_E$, used as the output signal in a transistor?

We know that the emitter current, $$\I_E\$$, is greater than the collector current, $$\I_C\$$. So, why isn't the emitter current used as the output signal in transistors for greater amplification?

• Have you seen the emitter follower circuits? Are you familiar with the common-base, common-collector, and common-emitter circuit configurations ?
– AJN
Jun 3, 2021 at 9:16
• How much larger is emitter current than collector current? 1.01 times? 1.1 times?
– AJN
Jun 3, 2021 at 9:17
• $I_E$ is very similar to $I_C$ so if my "amplification" is the current gain then we can use $I_E$ as output. Which is done in the common collector circuit configuration (also called emitter follower). For voltage amplification we cannot use $I_E$ as $V_{BE}$ is somewhat constant and that limits the voltage gain. So be clear what you mean by "amplification", voltage or current? Jun 3, 2021 at 9:19
• I actually have seen the emitter follower (common collector) circuits. Didn't notice that I_E is used as the output signal there. My bad. Well, I_E=I_B+I_C. So, I have asked the question on that premise that I_E is greater than I_C. Jun 3, 2021 at 9:19
• @user545735 There's nothing wrong with asking questions like this. It's good, in fact. So don't beat yourself up. In general, $I_E\approx I_C$. So there's little advantage if all you are looking at is current compliance. Also, the emitter "follows" the base (if you apply the signal to the base) and the collector inverts the base. So you can chose which way to go, as well. But because the emitter follows the base almost exactly, the voltage gain is close to 1. Because the collector can have a resistor stuffed in to convert the current to a voltage, the voltage gain can be large, but inverted.
– jonk
Jun 3, 2021 at 9:44

## 1 Answer

Three common BJT amplifier topologies are common emitter, common collector and common base. Each of these topologies has different characteristics in terms of input impedance, output impedance, voltage gain, and current gain.

Common emitter amplifiers use the base for input and collector for output. They have high voltage gain, but relatively high output impedance.

Common collector amplifiers (also called emitter followers) use the base for input and the emitter for output. They have virtually no voltage gain, but great current gain and very low output impedance.

Common base amplifiers use the emitter for input and the collector for output. They have virtually no current gain, medium voltage gain, but low input impedance.

If you want an amplifier with virtually no voltage gain, but good current gain and low output impedance, then the common collector topology, using the emitter for output is an excellent choice.