![]() It's both and, it's also temperature and, it's also circuit repeatability. Is it a change in supply voltage, or change in transistor beta? So, do you want a 5h1tty circuit that drifts around or, do you want a stable circuit?īut I'm struggling to understand the exact circumstance where this would be needed. If you didn't have an emitter resistor, how can you properly define the current flowing in the emitter (and the collector)? Well, you can if you know the maths surrounding a CE configuration but, then you'll find that temperature effects will be surprisingly high and the desired 6 volts at the collector may drift with temperature a few volts this way and that way. So, that's the normal case when an emitter resistor is used. So, it's easy to follow and you should nearly always get predictable results but you'll never get exact results from a single BJT amplifier. In this example I used a BC547 BJT and the base voltage to ground is 1.269 volts (implying that the base-emitter volt-drop is 0.669 volts). Here's a simulation where I've tweaked R4 to show what I mean:. Knowing this allows you to calculate the base-bias resistor values. So, around 1.301 volts on the base will ensure that there is approximately 6 volts DC at the collector. It then follows that the base-bias voltage would need to be about 0.601 volts plus about 0.7 volts (internal base-emitter volt-drop). If the emitter resistor is 220 Ω (for example), a DC current of 2.73 mA flows from the collector into the emitter and then through the resistor and, it would drop about 0.601 volts across it. To ensure that a DC current of 2.73 mA flows through Rc consistently requires an emitter resistor. ![]() So, if Vcc is (say) 12 volts and Rc is (say) 2200 Ω then you want the quiescent current through Rc to be about 2.73 mA (2.73 mA through a 2k2 resistor drops 6.006 volts). To achieve this, you need to create a situation where the DC quiescent collector current is held fairly constant. Another way of phrasing it is that you are aiming for a good linear dynamic range capability. So, when you have an AC signal present at the input, the amplifier's output (collector) doesn't clip the signal asymmetrically. What most folk want with a common emitter amplifier is for the DC collector voltage to be about half the supply voltage. ![]() I'm struggling to understand the exact circumstance where this would ![]()
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