Why do transistor transmitters not have base resistor connected to collector instead of vcc?

Question

I was researching here on transistor amplifiers:

http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html#Stage and they were talking about these two transistor amplifiers. I use the one on the left because my circuits function better.

When I built my regen receiver, it utilized negative feedback in all stages and the circuit operated fine. If however, I took the class C approach, then the regen wouldn't oscillate as well.

I then read on the site in regards to the circuit on the right:

This stage produces the maximum voltage amplification but it is very difficult to "set-up" because the value of the base resistor will either make the collector voltage nearly zero or full rail voltage. It is very difficult to get the collector to sit at mid rail. If the base resistor is a high value, the collector will sit at rail voltage. If the base resistor is a low value, the collector will sit a 0v. If a transistor with a different gain is fitted, the collector voltage will change completely. If it sits at mid-rail, the noise produced by the transistor will make the collector voltage rise and fall and produce a lot of noise.

In a transmitter, I want to be able to transmit the signal I want, not a bunch of random noise that a transistor produces.

I then go find a random transmitter design from the internet and resistor R4 is connected from base to VCC instead of from base to collector of the same transistor. Why is that?

Answer 1

The two circuits are identical as far as bias is concerned:

^{simulate this circuit – Schematic created using CircuitLab}

Let's take the oscillator, let's strip off all AC stuff, and finally note that being BAT1 and R5 series connected we can swap their relative order not affecting the rest.

Answer 2

I then go find a random transmitter design from the internet and resistor R4 is connected from base to VCC instead of from base to collector of the same transistor. Why is that?

This is just a slightly different way to provide negative feedback for Base bias. The difference is that the 'load' resistor is on the Emitter instead of the Collector, but the result is the same - as Q2 draws more current so R5 drops more voltage which leaves less voltage across R4, thus reducing Base current and stabilizing the operating point.

But why do it this way? For the RF oscillator function Q2 is operated in 'Common Base' mode, where the Base is grounded (to RF) via C3 and positive feedback is applied to the Emitter via C6. R5 is already required to develop the RF input voltage across, so it is 'free' for use in stabilizing the Base bias.

The RF oscillator operates in Common Base mode so that audio frequency FM modulation can be applied to the Base. Here Q2 is operating in 'Common Collector' mode, with R5 helping to provide the FM modulation function (which utilizes the change in capacitance that occurs when Q2's Collector-Base voltage changes).