Electronic – Simple version of an internal op-amp circuit

1. It's common-base. You can recognize it because the base is bypassed with large value capacitor to a supply rail or ground (it's at AC ground). Large is in relation to the operation frequency, which in this case appears to be about 100MHz.

2. C1 keeps the base grounded for RF, R1 provides bias when the input is open (or capacitively coupled).

3. L1 and C2 + C3 + Ccb are a resonant circuit. The output is taken from the collector. C3 provides feedback to the emitter, around half the voltage at the collector appears at the emitter. Without C3 it won't oscillate, or at least not very well. C2 allows the oscillation frequency to be determined separately from the amount of feedback.

Yes, generally the power supply has to be capable of supplying all the current. There are exceptions we'll look at later.

When we say "a transistor amplifies current" what we mean is that we can use a transistor to allow a small current to control the flow of a larger current. This is analogous to, for example, power steering where a weak turning effort on the wheel of the car or truck is amplified into a high force at the wheels. The power obviously has to come from somewhere and in our electronic amplifier it comes from the battery or power supply.

The exceptions I mentioned earlier include cases where a transistor is used as an interface between a low power circuit and another high power circuit. In this case the amplified current is coming from a different power supply.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A common emmiter transistor switch. A couple of milliamps flows from the 3.3 V micro into the base of the transistor which turns on and allows about 500 mA to flow through a 6 W lamp powered by another supply. Note the common ground connection between the two circuits.