BIASING - Page 1
Forum section: electronics / beginners

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 19, 2019, 11:17:08 am
content: 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=db3a304314dca389011518d610ec0e153. What is the purpose of the complementary transistors at this case?https://www.diodes.com/assets/Datasheets/ZXTC2062E6.pdfYou can find the full schematic herehttps://files.ettus.com/schematics/b200mini/b200mini.pdfThanks in advance!!

author: mikerj
author_rank: Super Contributor
timestamp: June 19, 2019, 12:28:28 pm
content: 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.

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 19, 2019, 01:14:21 pm
content: Thank you for your answer!!Quote from: mikerj on June 19, 2019, 12:28:28 pmI 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 pmHowever, what is their general purpose?

author: virtualparticles
author_rank: Regular Contributor
timestamp: June 19, 2019, 02:24:53 pm
content: 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.

author: mikerj
author_rank: Super Contributor
timestamp: June 19, 2019, 02:55:11 pm
content: Quote from: Nikos A. on June 19, 2019, 01:14:21 pmAt high frequencies parasitic inductance limits the minimum impedance of a capacitor. Three terminal capacitors minimise inductance.

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 19, 2019, 03:06:06 pm
content: Quote from: mikerj on June 19, 2019, 02:55:11 pmSo, as you mentioned a three terminal capacitor provides good decoupling performance. Why they are not used as usual as the common decoupling capacitors? I realize that they cost much more, but is this the real reason?

author: mikerj
author_rank: Super Contributor
timestamp: June 19, 2019, 04:24:18 pm
content: Quote from: Nikos A. on June 19, 2019, 03:06:06 pmFeedthrough capacitors tend only to be available in low capacitiance values, and decoupling capacitors in lower speed circuits don't need the high frequency performance.

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 20, 2019, 09:27:44 am
content: Thank you very much for your replies!! The only that is still confusing me, is how you achieve fast switching by using the anti-parallel diodes.Quote from: virtualparticles on June 19, 2019, 02:24:53 pmIs it possible to give me some more detail?

author: mikerj
author_rank: Super Contributor
timestamp: June 20, 2019, 09:55:00 am
content: C1/C2 and R2 form a low pass filter. Ignoring the diodes, if a fast edge is applied to to cTXDRV-PWEN then the voltage across C1/C2 (and therefore the base/emitter of the NPN transistor) will be an inverse exponential rise with time, slowing down the turn on (or off) of the transistor.With the diodes in place, when the voltage across R2 exceeds about +-0.3v (schottky diode) one of the diodes will conduct and pass more current into C1/C2 increasing the rate of change of voltage and speeding up the switch on of the transistor. When the capacitors have completed charging (or discharging) there will be no voltage drop across R2, and the diodes will be turned off. In this state any noise up to the +-0.3v threshold will be filtered by R2 and C1/C2.

author: Nikos A.
author_rank: Regular Contributor
timestamp: June 20, 2019, 10:25:13 am
content: Thank you for your great explanation!!!

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 21, 2019, 07:13:09 am
content: Quote from: virtualparticles on June 19, 2019, 02:24:53 pmSo you mean as a way to bypass the resistor and allow the cap to drain or charge through the diode when the state changes ?OP it's also a 'clipper circuit' when ,back-to-front diodes are across something, they would try and limit the max voltage across the resistor to the diode's voltage drop...until they burn out.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2019, 09:51:18 am
content: Quote from: Nikos A. on June 19, 2019, 11:17:08 amVss is shorthand for the substrate connection which in a common junction isolated process, must be at the most negative potential or lower. So usually this connection is made to ground or the negative supply but it may or may not be common for the IC.QuoteI think those are intended to speed up switching of the level shifter so power may be applied or removed more quickly without waiting for the 10 kilohm resistor to charge or discharge C1 and C2. But I think this could have been designed better if that was the goal.QuoteThey are configured as a level shifter to power the amplifier when enabled by a logic level signal.