I'm reading Electronics for Dummies. So far it's great for a complete beginner, however, I've come to a chapter about transistors and either I've missed something or there's a sudden jump in complexity.

In the circuit below:

  1. Why is Vout connected (via C2) to the collector of the transistor and not the emitter? My understanding is the current would flow from the collector, to the emitter, amplifying the signal that's come through the base. In this case, it seems like it would just pick up the current from +VSupply through R4.
  2. What's the purpose of R1 and R3? It looks like these go to ground, which in this case, I assume would be the negative terminal of the battery. I understand they affect the current of the whole circuit and that the current doesn't just "slow down" going through a resistor, but why are they there before ground? Is it for convenience and they could go somewhere else?
  3. I don't understand what happens at the junction connecting R2, R1 and C1 to the base of the transistor. At this point, I assume current is flowing from +Vsupply, but there's also a tiny current flowing through C1 from vin - so what happens when these two currents meet? I think maybe I'm confusing current as a result of voltage and as opposed to an electrical "signal".
  4. Also on the subject of that junction, why doesn't the current caused by the voltage of +Vsupply flow the wrong way into C1? Is it because the current is trying to "find" ground and so goes to the most direct path?

Basic amplifier

  • \$\begingroup\$ Most of your questions can be resolved by understanding that the capacitors provide a way of using a circuit that can only handle voltages above 0 V to amplify a signal that oscillates above and below 0 V. C1 blocks the DC bias feeding back to the signal source. C2 prevents the DC bias affecting the following circuit. \$\endgroup\$
    – Transistor
    Sep 1, 2021 at 6:25
  • \$\begingroup\$ 1) You can take the output from emitter also, it is not inverted like collector signal is (advantage). But R3 is ussually designed smaller than R4 so smaller voltage appears on it. You can design it larger if you want but you get smaller gain. 2) R3 is not necessary, it is used as emitter degeneration, as I mention it reduce gain. It makes some kind of negative feedback to the base, so if the Beta (biasing) changes it reduce an error at the output. \$\endgroup\$
    – user208862
    Sep 1, 2021 at 6:48
  • \$\begingroup\$ Try read this electronics.stackexchange.com/questions/355899/… \$\endgroup\$
    – G36
    Sep 1, 2021 at 14:18

1 Answer 1

  1. You could connect C2 to the emitter, but R3 is usually a much lower value than R4, so the output signal would be less.

  2. Firstly, ‘earth’ is a concept. It is a reference point rather than something physical. If the circuit was powered by a battery, then most likely it would go to the - on the battery. In some instances it might be the +. Nevertheless it is a point of reference. Why R1and R3? The current has to flow somewhere and we use these resistors to set the operating point of the transistor. It all comes back to Ohms law and that current flows in a loop.

  3. What happens when two currents meet? You get the sum of the two currents. What the resistors are doing is setting the operating point of the transistor so that the collector is at half the voltage of the supply. R3 sets the gain - how much the transistor will amplify. C1 couples the input signal into the base. DC voltage cannot flow through a capacitor, only AC. Since audio is an AC signal, that tickles the base. We then ser the amplified signal on the collector. C2 ensures DC wont pass to the output, only the amplified AC.

  4. Again, capacitors block DC, so the supply voltage does not flow through C1.

It seems you’ve skipped a bit of theory, so that’s why you have these questions. Normally you’d start with DC theory like Ohms law and Kirchoffs law, then move onto sone basic AC theory then onto basic transistor physics then apply DC theory to transistors and so on. This is first year engineering. There’s free online courses that take you through this and beyond. If you’re serious then I’d suggest this rather than electronics for dummies. In a similar vein, my daughter asked me to teach her calculus. She has only started learning algebra so i told her it would make not much sense until she learnt a number of other concepts that lead to understanding calculus. Sometimes there is no short cut.


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