Some questions about the emitter resistor added to the fixed bias circuit

In many texts it is mentioned that the emitter resistor stabilizes the bias circuit. For example particularly for the below example:

Which quantity is aimed to be stabilized initially? Vbe?

And how does adding Re archives this? Can you give an example scenario so we can see that how Re works?

• RE does not stabilize Ib in the case you have shown. You'd need a resistive divider providing Vb-Ib to get the feedback effect of RE. Typically in that situation the divider current would be at least 10x the required base current. – Jack Creasey Jun 16 '18 at 16:05
• Imagine Vcc varies a bit. If there would be no Re, wouldn't Vbe be less stable to the variations of the Vcc? – panic attack Jun 16 '18 at 16:08
• The shown circuit is not a good one. The resistor RE provides current-controlled VOLTAGE feedback - however, the basis is nor biased with a voltage (as stiff as possible) but with a current. In "Art of Electronics" you can read: "Dont do this"! – LvW Jun 16 '18 at 17:11
• Re doesn't allow Vbe to increase too much... If emitter voltage goes down current through Re increases which makes the voltage go back up (negative feedback) and vice versa. – sarthak Jun 16 '18 at 22:00

Assume there is no emitter resistor.

Increase in VCC will lead to increase in collector current, also base current. This may have negative effect. For example, the temperature will now increase because of higher current.. As temperature increases, Vbe of a transistor reduces further..

As Base emitter voltage drops, base current would gain further triggering rise in collector current and this will make the circuit less stable.

Adding the emitter resistor counter acts by slightly opposing the rise in collector current. Whenever there is a rise in collector current, the voltage drop across the emitter resistor increases. This voltage drop now acts as a negative feedback. Higher the drop across the resistor value, lesser will be the collector current due to lesser base current.

As VCC increases, voltage drop across emitter resistor increases and similarly when VCC falls, the voltage across emitter resistor falls enabling higher collector current due to increase in base current.

Ideally I would use resistor divide bias at this base side..and keep base current only a fraction of current flowing through to bias resistors.

• You explained the logic very good. My problem is every text book I try do not tell in detail these and just say "Re stabilizes" ect. without explaining the logic behind. – panic attack Jun 16 '18 at 23:57
• @panicattack For base current stability, search fro resistor divide bias.. For that adding a emitter resistor will keep circuit stable – Umar Jun 17 '18 at 4:35
• I must admit that I do not agree to the "logic" of Umars explanation. He starts with a drop of the base-emitter voltage - however, in fact this will be the RESULT of an increase of the emitter current - mostly caused by a temperature increase (VE goes up, VBE goes down). My recommendation: Read and understand the contribution of Spehro Pefhany. – LvW Jun 17 '18 at 7:29
• Umar, you have mentioned "base current stability". Is this really your goal? For a constant and stabilized base current (how will you achieve this?) the collector current will strongly depend on temperature and beta-variations. Is this want you want? – LvW Jun 17 '18 at 7:33
• My assumption and suggestion is to go for resistor divide bias and my explanation for that – Umar Jun 17 '18 at 7:38

In this circuit, it doesn't stabilize anything. Assuming a Vcc that is 5 or 10V, the bias is dominated by the hFE of the transistor, which is quite variable from unit to unit and changes significantly with temperature. The -2mV/K Vbe variation is relatively unimportant.

In fact it probably makes it worse because there is less swing available at the collector, if you defined what the edges of acceptable operation were you would see that.

If you replace Rb with a voltage divider Re will help.

In this circuit, the Re does assist in stabilizing Ic

simulate this circuit – Schematic created using CircuitLab