I am a beginner in Electronics; I am learning the basics and am trying to take up circuitry as a hobby! My query is regarding power supply requirement for any circuit.
Take, for example, a circuit to light an LED. I can tell the supply voltage by knowing the LED's Forward Voltage Drop, its maximum current rating, and the resistor in the circuit. By choosing a different valued resistor (or a different LED, perhaps), I can choose a different supply voltage.
Now consider this circuit. What changes needs to be made to use a different supply voltage? More specifically:
- Is it only resistors which matter?
- How to know which resistors to change out to use a different supply voltage and which ones to leave alone?
On that note, how come some circuits have a fixed supply voltage while others have flexible ones (see this)? I know that I can't light an LED with anything between 1.5V and 9V without making any changes to the circuit!
I am not asking about voltage converters/regulators; see the LED lighting example above.
I have been stuck on this for quite some time now, reading articles and tutorials, but I can't seem to find anything which answers my query. Any help would be appreciated.
Best Answer
Thus clever little circuit is fairly stable over a wide range of voltages as a toy FM transmitter.
R2C3 act as a sawtooth ramp generator but the frequency is controlled by the high Q resonator tunes by L1 C4 with D5,C7 and to some degree C6 and the antenna.
So a tiny sawtooth generator only 2mV-pp occurs on the base with the Colpitts Oscillator getting pumped by the emitter-collector current pulses.
Due to C ratios in the oscillator the emitter generates about Vcc/2 in Vpp AC near 100 MHz while the collector tied to Vcc with the coil produces 2x Vcc in Vpp AC (doubler) just by ringing when it gets pulled down by the collector. Which occurs whenever Vbe exceeds 600mV which is just the peak of the sinewave. As it switches on, the collector pulls a few milliamps for < 1% of the sinewave and along with it the base voltage to turn it off again. So this is a very stable LC-tuned Relaxation oscillator. The audio then modulates this base voltage to change the current and shift the frequency ever so slightly 75kHz/100MHz max. Analog circuits depend on voltage or current references and better ones are regulated by precision resistor ratios with feedback so that over a range of supply voltages, they are stable. Others are optimized over a narrow range for efficiency and must have low noise well-regulated linear supplies. To change the voltage often is just changing resistor ratios to keep the same base bias voltage which results in emitter/collector current limiting as above.
Thus above one could reduce the voltage in half by reducing the base pull-up R2 to keep Vb constant so that Re = R3 keeps the same DC current.