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DigiTan said:Okay, I guess those engineering types really liked their greek letters. :lol:
I want to be 100% sure I understand exactly what 'saturation' means. The way I gather, a NPN transistor is saturated with it's base voltage is greater than its collector voltage (typically by 0.4V?). Otherwise, it's in the "active" mode. And a NPN with its base grounded is in the "cutoff" mode.
Is all this looking on-target so far?
You cannot cram any more current into a transistor's base-emitter junction when the collector voltage is so low that there is forward-biased base to collector current! That's when most transistors are saturated at low collector current.DigiTan said:I want to be 100% sure I understand exactly what 'saturation' means. The way I gather, a NPN transistor is saturated with it's base voltage is greater than its collector voltage (typically by 0.4V?).
audioguru said:You cannot cram any more current into a transistor's base-emitter junction when the collector voltage is so low that there is forward-biased base to collector current! That's when most transistors are saturated at low collector current.
DigiTan said:So for low-distortion gain, I would need to bias the transistor into the active mode, right?For low distortion in a transistor common emitter transitor circuit, keep the output voltage well away from the supply voltage and use plenty of negative feedback by using an unbypassed emitter resistor or other ways. I attach a sim of a transitor producing about 40 percent distortion!
With the emitter resistor unbypassed, the distortion is only (!) 3%. The distortion is lower at lower levels.
I have seen Mosfets and Jfets as linear resistors in the saturation mode where there isn't a DC supply and the signal across them is 100mV or less. Usually they are used in their active triode mode with plenty of negative feedback to reduce distortion. :lol:Does the opposite case apply to MOSFETs? In our examples, we always had the MOSFET in saturation mode because the triode mode had an exponential I-V characteristic that distorted the signal--whereas saturation was mostly linear. Is this part correct?
The gain is slightly lowered, and it depends on resistance. Distortion and gain tend to inverse each other.DigiTan said:Okay, so bypassing the Re causes the gain to be increased, but causes distortion because the transistor was driven into saturation. And when the resistor can't be bypassed, the gain stays low. Is this part correct?
To me, yes. saturating the transistor is like flooding a car :lol:isn't saturation simply when the transistor is turned all the way on?
No. It isn't saturated and it isn't cutoff. It is just very badly distorted because transistor circuits as simple as this are lousy.DigiTan said:Okay, so bypassing the Re causes the gain to be increased, but causes distortion because the transistor was driven into saturation.
Correct. But I increased the input level about 18 times so that both have the same output level.And when the resistor can't be bypassed, the gain stays low. Is this part correct?
No. Bypassing the emitter resistor increases the gain 18 times or more. Since the resistor is bypassed with a 100uF capacitor that is like a dead short to the signal at the emitter, then the transistor doesn't have any negative feedback. Negative feedback reduces gain and distortion and increases bandwidth.mstechca said:The gain is slightly lowered, and it depends on resistance.
No they aren't, look at my pics. The one with high gain also has very high distortion. Negative feedback reduces gain and distortion.Distortion and gain tend to inverse each other.
A voltage-follower doesn't have any voltage gain.DigiTan said:why is the CE amp typicaly seen in the inverting configuration and not the voltage-following mode? Is it because of performance or is it just the convention?
I was making an assumption that no bypass capacitor was used.audioguru said:No. Bypassing the emitter resistor increases the gain 18 times or more. Since the resistor is bypassed with a 100uF capacitor that is like a dead short to the signal at the emitter, then the transistor doesn't have any negative feedback. Negative feedback reduces gain and distortion and increases bandwidth.mstechca said:The gain is slightly lowered, and it depends on resistance.
I forgot to add gain.No they aren't, look at my pics. The one with high gain also has very high distortion. Negative feedback reduces gain and distortion.Distortion and gain tend to inverse each other.