Electronics > Projects, Designs, and Technical Stuff
Level shifting
Zero999:
--- Quote from: Benta on January 08, 2019, 03:51:08 pm ---
--- Quote from: syntax333 on January 08, 2019, 12:37:01 pm ---What is downside of large Re? You said that it is bit high, how did you decide that? I am familiar with regular emitter follower circuit however I have never used them in this configuration.
While I was researching I found that this configuration is used mainly to get rid of the dc offset, however I couldn't find any source that shows design procedure.
Can you explain or give reference for this circuit so I can learn more about this configuration?
--- End quote ---
The first Re supplies base current to the PNP emitter follower. If you make it 1 Mohm you'll starve the transistor completely of base current.
The complementary emitter follower quite correctly conserves the DC level of the signal. A simple emitter follower shifts the signal by Vbe (~0.65 V). By having and NPN and a PNP stage, this is eliminated.
It's not my invention :) I first noticed the circuit many years ago while repairing VCRs and shamelessly copied it for my own designs.
--- End quote ---
It's a fairly common circuit. A push-pull variant is often used for audio amplifier output stages, which will probably work well. It can be improved by adding current sources/sinks.
Refer to (b)
https://www.researchgate.net/figure/Different-kinds-of-class-AB-output-stages-a-Push-pull-network-and-b-improved_fig1_3139443
Benta:
Zero999, my apologies. I used an incorrect term for the buffer stage.
It's not a "complementary emitter follower" which your pictures correctly show, but an "NPN - PNP emitter follower cascade".
Very often found in CVBS applications where no voltage gain, but DC-level conservation and current gain is needed.
Zero999:
--- Quote from: Benta on January 08, 2019, 05:06:04 pm ---Zero999, my apologies. I used an incorrect term for the buffer stage.
It's not a "complementary emitter follower" which your pictures correctly show, but an "NPN - PNP emitter follower cascade".
Very often found in CVBS applications where no voltage gain, but DC-level conservation and current gain is needed.
--- End quote ---
Yes, but it works on the same principle. Remove Q1, Q4 and IQ1 and replace IQ2 with a resistor and it's the same circuit as yours.
The complementary version will be better, but it's more complex.
Benta:
--- Quote from: Zero999 on January 08, 2019, 05:13:48 pm ---The complementary version will be better, but it's more complex.
--- End quote ---
I'm not certain about that. A video buffer should have a GBW of at least 100 MHz to avoid colour carrier phase shift. The simple emitter followers will do this, but with the more complex types you posted I have my doubts (parasitics).
Zero999:
--- Quote from: Benta on January 08, 2019, 05:40:59 pm ---
--- Quote from: Zero999 on January 08, 2019, 05:13:48 pm ---The complementary version will be better, but it's more complex.
--- End quote ---
I'm not certain about that. A video buffer should have a GBW of at least 100 MHz to avoid colour carrier phase shift. The simple emitter followers will do this, but with the more complex types you posted I have my doubts (parasitics).
--- End quote ---
Why do you believe the complementary version will have a lower bandwidth, than the single ended one? It should have a lower output impedance. The main issue I see is crossover distortion.
The input impedance goes down of both circuits, with increasing frequency, so the bandwidth will be limited by the source and load impedances. The 10k resistor will limit the bandwidth in your circuit. What does it do? If it's removed, the bandwidth will increase.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version