| Electronics > Beginners |
| BIASING |
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| Nikos A.:
Hi everyone, I am troubleshooting a circuit and I've got some questions regarding the schematic. I attach a screenshot presented the biasing circuit of a LNA. 1. Why the designer uses VSS to indicate ground? Is there any specific reason? 2. What is the purpose of anti-parallel diodes at this case? https://www.infineon.com/dgdl/Infineon-BAS125SERIES-DS-v01_01-en.pdf?fileId=db3a304314dca389011518d610ec0e15 3. What is the purpose of the complementary transistors at this case? https://www.diodes.com/assets/Datasheets/ZXTC2062E6.pdf You can find the full schematic here https://files.ettus.com/schematics/b200mini/b200mini.pdf Thanks in advance!! |
| mikerj:
The PC105 component is a passthrough capacitor. This is a three terminal capacitor designed to give good decoupling performance at high frequency. The VSS label appears to be just the preference of whoever drew the schematic. More usually this label is used to label a negative rail. The anti-parallel diodes are slightly puzzling, I can only guess that they are used to speed up the switching of Q1 (by bypassing R2) whilst keeping the steady state base current low. Q1 is simply a power switch controlled by the cTXDRV-PWEN signal, to provide power and bias to U2. |
| Nikos A.:
Thank you for your answer!! --- Quote from: mikerj on June 19, 2019, 12:28:28 pm ---The PC105 component is a passthrough capacitor. This is a three terminal capacitor designed to give good decoupling performance at high frequency. --- End quote --- I really didn't even know the existence of such a component. What is the difference of using a simple capacitor for AC decoupling? --- Quote from: mikerj on June 19, 2019, 12:28:28 pm ---The anti-parallel diodes are slightly puzzling, I can only guess that they are used to speed up the switching of Q1 (by bypassing R2) whilst keeping the steady state base current low. --- End quote --- However, what is their general purpose? |
| virtualparticles:
Diode D1 is indeed there to allow fast logic switching while R2 and C1 and C2 filter the noise. Noise voltage is assumed to be low enough to not turn on the diodes but a real logic signal will turn on the diode and fly over R2. VSS usually stands for the lower potential, ground or negative in a FET circuit Where VDD is the higher potiential. (Source.. Drain) VCC is usually the collector voltage in a BJT circuit where VEE would be the lower potential. (Collector... Emitter) Q1 is is a switching circuit for the power supply of U2. A high level at TXDRV-PWEN turns on the lower transistor which then pulls down the base of the upper transistor, turning it on and causing the +3.3V to appear on its collector. I would normally put a pull-up resistor on the base of the upper transistor going to +3.3V (maybe 10K) to better define that node instead of relying on the leakage through the transistor but I suppose it isn't absolutely necessary. |
| mikerj:
--- Quote from: Nikos A. on June 19, 2019, 01:14:21 pm ---Thank you for your answer!! --- Quote from: mikerj on June 19, 2019, 12:28:28 pm ---The PC105 component is a passthrough capacitor. This is a three terminal capacitor designed to give good decoupling performance at high frequency. --- End quote --- I really didn't even know the existence of such a component. What is the difference of using a simple capacitor for AC decoupling? --- End quote --- At high frequencies parasitic inductance limits the minimum impedance of a capacitor. Three terminal capacitors minimise inductance. |
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