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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.

amp with negative feedback base with pull-up resistor

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?

sample transmitter

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    \$\begingroup\$ The short answer is - R5. The provides what's called emitter degeneration, and permits stable operation. Note that there is no such resistor in your examples. \$\endgroup\$ – WhatRoughBeast Sep 11 '16 at 14:30
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The two circuits are identical as far as bias is concerned:

schematic

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.

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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).

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  • \$\begingroup\$ I'm trying to understand this... I know that I'm using the NPN in common-base and that the capacitor from base to ground is shorted (ac wise) at VHF but why is it advantageous to connect resistor from base to VCC instead of from base to collector? wouldn't the oscillator be easier to start up and less power be drawn if i connected the resistor from base to collector? I'm trying to figure out which method is more advantageous and why. According to talkingelectronics.com, connection resistor from base to VCC gives the transistor the opportunity to make noise. \$\endgroup\$ – user116345 Sep 11 '16 at 15:25
  • \$\begingroup\$ The Collector is connected to Vcc via the tank coil. At DC this is just a short piece of wire, so the resistor effectively is connected to the Collector. You could connect it directly to the Collector, which would do the same job but put extra load on the tank circuit, reducing its Q and making the oscillator less stable (perhaps not by much, but why do it when you don't have to?). \$\endgroup\$ – Bruce Abbott Sep 11 '16 at 17:47
  • \$\begingroup\$ "According to talkingelectronics.com, connection resistor from base to VCC gives the transistor the opportunity to make noise." - where do they say that, and why do they still do it? talkingelectronics.com/projects/FM-Bug/FM-Bug.html \$\endgroup\$ – Bruce Abbott Sep 11 '16 at 17:49
  • \$\begingroup\$ I read it under "A stage" in talkingelectronics.com/projects/TheTransistorAmplifier/…. "If it (the input?) sits at mid-rail, the noise produced by the transistor will make the collector voltage rise and fall and produce a lot of noise." \$\endgroup\$ – user116345 Sep 11 '16 at 18:15
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    \$\begingroup\$ That's not right (and doesn't apply to your circuit anyway, since it has an Emitter resistor). The only reason biasing directly to Vcc is bad is that there is no bias stabilization, so for accurate biasing the resistor must be trimmed to match each individual transistor. However the lack of negative feedback does let the transistor produce maximum gain, so any noise at the input will be amplified more (as will the signal, so no difference in Signal/Noise ratio). \$\endgroup\$ – Bruce Abbott Sep 11 '16 at 19:17

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