Common Collector - PSpice AC Analysis - Page 1
Forum section: electronics / projects

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: December 09, 2020, 08:14:42 pm
content: Hello everyone, I have been given the assignment to design an LED driver using a BJT(or MOSFET or combination, I cannot use opamps). The circuit is supposed to act as a buffer and adhere to the following specs listed below.1) High input impedance --> Thus the common collector.2) High current gain3) To be able to drive the led using the Philips RC‐5 protocol.The LED I have been given to work with is the tshg8400 datasheet:https://www.vishay.com/docs/81297/tshg8400.pdf.I modeled most of the parameters of the LED needed for the AC analysis (some I left untouched). (picture below)So far I the circuit is behaving amazingly in DC. (Without the output capacitor)My issue begins when I try to do an AC analysis on the circuit. When the output capacitor is added, almost no current flows through the diode.The voltage gain as expected of a buffer circuit follows the input voltage but I cannot figure out why the current gain is so low.As for the frequency part of the protocol the values of the input and output capacitor I will change later to get the needed response.I would love your feedback on that because I could not find a solution to this still.I am not sure about my analysis parameters so I posted them as well just to be safe.Thank you in advance.

author: Benta
author_rank: Super Contributor
timestamp: December 09, 2020, 08:39:02 pm
content: Why would current flow through the LED when you have a series capacitor?The DC voltages on both sides of the cap would simply adjust, so that no current flows.Just for fun, add a diode in anti-parallel to the LED and see what happens. You'll need to reduce R2 for this to work.

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: December 09, 2020, 09:08:16 pm
content: Well yeah, I expected not to have a DC voltage due to the capacitor but what about the AC signal I need to be amplified. Also, I am starting to believe that something is wrong with my simulation parameters (dropped R2 down to 1K and added the diode but the waveform stayed the same).Edit: I believe I set up something wrong in my simulation. I tried using everycircuit to design the exact same thing and I was getting the expected output.

author: Benta
author_rank: Super Contributor
timestamp: December 09, 2020, 11:43:03 pm
content: Quote from: ChrisGreece52 on December 09, 2020, 09:08:16 pmWell, I'm an old school guy sketching on paper with a pencil when designing. But I don't even need to do that to see that your voltages are completely off.Perhaps you should try the pencil/paper trick before firing up the fantastic simulator? It'll give you a picture of the realistic values.

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: December 10, 2020, 10:40:14 am
content: Thank you for your reply. I am confused about what the voltages should be. I am working with the 2n2222.When I did the DC analysis I only had in mind to bias the transistor correctly so it outputs the signal properly.Now that I saw the circuit again I got why the output current was minuscule (in DC at least did not bother with AC given the DC results).When designing a transistor circuit from scratch, what would be the procedure? By that I mean, what should I focus on first?It's a piece of cake doing calculations on a circuit and figuring out what the voltages and currents are and determining the behaviour of the circuit, but when it comes to designing I haven't gotten the hang of just plopping down a transistor and figuring the other things down.Edit: I tried starting with the calculations on paper but got lost pretty quickly and decided to just start the simulation not to run out of time. But since that hasn't helped me at all ill stick to paper for now.Edit 2: I started by setting the goals for the amplifier. My goal is to get 200mA on the emitter. For that to happen the Vce to be at around 12V the emitter resistor Re must be 65 Ohms. Even in AC where the capacitor would not be an open circuit the current flowing to ground.Edit 3: I literally think I have no idea what I am doing anymore. I have no idea why but I am getting proper current flow through the capacitor @ 1GHz. Ill have to start from scratch and figure it out.