I'm designing an RC phase shift oscillator. (Although, I'm faced with the same issue when designing a Colpitts oscillator.)
I'm confused about what looks, to me, like a conflict between achieving the required amplifier gain and the Q point for VCE.
The amplifier uses a Common emitter Class A amplifier using:
NPN BJT, voltage divider bias, emitter bypass capacitor.
VCC of 14.1 V, 1 mA emitter (collector) current.
All the voltages and currents for the amplifier portion are as expected for a common emitter Class A amplifier.
Given what I think I understand about a DC load line and Q points, the Q point for VCE should be about 7 V. (Which my circuit achieves.)
Here's where I'm confused:
If I use the known, required amplifier gain of 29 and the collector current of 1 mA (keeping in mind that I'm using a emitter bypass cap), the calculated collector resistance should be:
r prime e = 26 mV divided by collector current = 26 mV divided by 1 mA = 26 ohms.
RC = Av times r prime e = 29 times 26 ohms = 754 ohms.
Using a resistor with a value anywhere near the 754 ohms will result in a voltage drop across RC of less than 1 volt.
This does not allow for much of a peak to peak signal voltage.
Plus, this resistance differs greatly from the resistance calculated via the DC load line approach.
So, what am I missing? What formula should I use to calculate collector resistance? (I'm not ready to use an RF choke, and the tutorials I've listened to say that a collector resistor can be used for both an RC and a Colpitts oscillator.)
Please do not use any images in your explanation. I am blind, so I can't access images.
Thhanks,
soc7
I'm confused about what looks, to me, like a conflict between achieving the required amplifier gain and the Q point for VCE.
The amplifier uses a Common emitter Class A amplifier using:
NPN BJT, voltage divider bias, emitter bypass capacitor.
VCC of 14.1 V, 1 mA emitter (collector) current.
All the voltages and currents for the amplifier portion are as expected for a common emitter Class A amplifier.
Given what I think I understand about a DC load line and Q points, the Q point for VCE should be about 7 V. (Which my circuit achieves.)
Here's where I'm confused:
If I use the known, required amplifier gain of 29 and the collector current of 1 mA (keeping in mind that I'm using a emitter bypass cap), the calculated collector resistance should be:
r prime e = 26 mV divided by collector current = 26 mV divided by 1 mA = 26 ohms.
RC = Av times r prime e = 29 times 26 ohms = 754 ohms.
Using a resistor with a value anywhere near the 754 ohms will result in a voltage drop across RC of less than 1 volt.
This does not allow for much of a peak to peak signal voltage.
Plus, this resistance differs greatly from the resistance calculated via the DC load line approach.
So, what am I missing? What formula should I use to calculate collector resistance? (I'm not ready to use an RF choke, and the tutorials I've listened to say that a collector resistor can be used for both an RC and a Colpitts oscillator.)
Please do not use any images in your explanation. I am blind, so I can't access images.
Thhanks,
soc7