Common Emitter BJT amp phase - Page 1
Forum section: electronics / beginners

author: TheBaconWizard
author_rank: Regular Contributor
timestamp: June 19, 2018, 07:45:23 am
content: I've been looking at common emitter BJT amplifiers and I get the principle in general.. I want voltage amplification hence this choice, and in this case it only needs to be on or off, so saturation-mode.However, all the tutorials I've seen so far tell me that the same waveform is produced whether I use NPN or PNP. It seems to me that they should be the precise inverse of each other? What am I missing?In fact.. if indeed they DO have exactly the same result, how WOULD I produce a signal that is exactly 180deg out of phase compared with these simple amplifiers?Many thanks

author: David Hess
author_rank: Super Contributor
timestamp: June 19, 2018, 09:17:03 am
content: Common emitter, aka shunt feedback amplifiers, invert the input signal whether NPN or PNP. If you want a non-inverted signal, then you can use a common base or common collector NPN or PNP, use two common emitter amplifiers in series, or maybe add a transformer.

author: TheBaconWizard
author_rank: Regular Contributor
timestamp: June 19, 2018, 10:51:07 am
content: Many thanks

author: vk6zgo
author_rank: Super Contributor
timestamp: June 19, 2018, 11:20:22 am
content: Quote from: David Hess on June 19, 2018, 09:17:03 amActually, you would use two CE stages incascade.Two transistors inseriesis a very different circuit configuration.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2018, 02:58:41 am
content: Quote from: vk6zgo on June 19, 2018, 11:20:22 amThanks. Learning or being reminded of something like this is why I participate.A differential pair can also be used and it produces both outputs.

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2018, 08:37:46 am
content: Quote from: TheBaconWizard on June 19, 2018, 07:45:23 amAn NPN transistor needs a positive base voltage to turn on and the opposite is true for a PNP device, so one might think it would produce the opposite, until one looks at the schematic. In the case of a common emitter amplifier, when the transistor is off, the output is high, because current flows to the output, via the emitter resistor and when the transistor is turned on, current flows through the transistor, bypassing the resistor, so the output is low.Now, compare the two schematics. The NPN transistor is wired, with its emitter at 0V and collector to +V, via a resistor and the PNP transistor is wired, with its emitter to +V and collector to 0V, via a resistor. When input2 is connected to 0V, the PNP transistor is on, connecting output2, directly to +V. Then input2 is connected to +V, the PNP transistor is off and the output is connected to 0V, via the resistor.So why use one circuit over the other? They have different characteristics.The NPN inverter is activated, when the input is above 0.6V and with the PNP inverter, it's activated when the input falls 0.6V below the supply voltage.When the inputs float, i.e. aren't connected to anything, the NPN circuit defaults to high and the PNP circuit to low, because the transistors are off and all that's left is the output connected to +V or 0V (depending on the circuit), via a resistor.When the transistor turns on, the output has a very low impedance, and when the transistor is off, the output impedance is simply equal to the collector resistor. The NPN circuit has a low impedance, when its output is low and the PNP circuit has a low impedance, when its output is high.