# Emitter Resistance in BJT Amplifier

What is the importance of resistance at emitter and collector of BJT amplifier? What will be effect on output if they are removed?

This is the basic circuit you seem to be asking about. It's known as a common-emitter amplifier with emitter degeneration: The two resistors have totally different functions.

RC is totally fundamental to the operation of a common-emitter amplifier. It determines the voltage gain of the amplifier.

If you removed it, you simply wouldn't have a working circuit, because there'd be no path for current to flow through the collector of the BJT. If you removed it and replaced it with a short circuit, the BJT would still produce current gain, but the output voltage would always be exactly equal to the V+ voltage, and the circuit just wouldn't be very useful.

Note: In some cases, RC is not present, but the load is connected from the collector to the positive supply, so that the load itself fills the role of RC.

RE, on the other hand, is a little more complicated. This resistor is why we call the circuit "emitter degenerate". Having RE means that an increase in collector current tends to reduce Vbe, which reduces the portion of the input voltage that contributes to gain. This is a form of negative feedback. The main benefits of this is that it increases the range of input bias where the circuit operates linearly, makes the circuit gain more stable if the BJT properties vary, and it increases the input resistance of the circuit.

If you removed RE and replaced it with a wire you'd just have a standard common-emitter amplifier.

• With the circuit as shown, changes in the Vin voltage will translate somewhat linearly into changes in Vout voltage when it's in the range roughly 0.7V to Vsupply * Re / (Rc+Re) + 0.7V. In the absence of the emitter resistor, unless one adds a base resistor, there won't be a useful linear range. – supercat Apr 11 '13 at 18:27
• @supercat, good point. Edited. – The Photon Apr 11 '13 at 18:52

The emitter resistor serves many functions.

1. Together with the biasing point used with the base, it sets the quiescent current for the amplifier. (In this sense, think of the BJT operating as an emitter follower.) The quiescent current will be $I_q\approx \cfrac{V_b-V_{be}}{R_e}$ (when $I_q\cdot R_E\ge 200\:\textrm{mV}$.)
2. Together with the collector resistor, it makes the estimated signal gain expected at the collector more predictable and reduces dependence of the specific $\beta$ of the transistor. The signal gain will be $\approx -\cfrac{R_c}{R_e}$ ($180^{\circ}$ out of phase with the input signal at the base.)
3. Reduces temperature dependence of the signal gain. The thermal voltage divided by the emitter current (sometimes called "little re" or $r_e$) represents an equivalent resistance that is added in series with the emitter resistance to compute signal gain. When $R_e\gg r_e$, changes in ambient or operating temperature have far less impact on the signal gain.
4. It similarly makes the quiescent current very much more predictable because it reduces the dependence upon $r_e$.
5. It increases input impedance two ways: (1) by raising the emitter voltage, the base voltage can be placed more to the center between the power rails, allowing the Thevenin equivalent of the biasing base pair of resistors to be higher; and, (2) reduces the impact of the reflected impedance seen at the base, since its effective impedance contribution is $\beta\cdot R_e$ and the larger that $R_e$ is, the less it impacts the biasing pair's Thevenin loading.
6. It also allows the emitter to provide a low-impedance output signal at a gain of about +0.99 (in phase with the input at the base.)

In fact, it serves so many different purposes that most practical amplifiers will find ways to separate some of the functions so that they can be independently set. For example, using an AC gain-setting leg in the emitter allows separating out the DC setpoint for the emitter voltage from the amplifier's signal gain. And bootstrapping, by using the low-impedance emitter output (in phase) to stiffen the base is also commonly seen.

the resistance in collector will determine the value of your V(CE) which is important for you to determine your quiescent point for your transistor depending on what is your purpose on your transistor. The resistance in emitter is for stability, because BJTs are highly sensitive in temperature and it will affect your current gain. Having a resistance in emitter will make your circuit more stable than no resistor at emitter. But you can still remove the that resistance in your emitter. Removing the resistor in your collector may cause damage to your transistor due to very high current.

• Remove the resistor in the collector and take the output from the emitter and you have a (useful) emitter follower. High current gain, voltage gain a bit less than 1. – Spehro Pefhany Sep 29 '14 at 15:07