Circuit Component, Cap question - Page 1
Forum section: electronics / beginners

author: Dellyjoe
author_rank: Regular Contributor
timestamp: March 18, 2020, 08:13:06 pm
content: Good Afternoon everyone,See circuit below Note the simple Cascade Amplifier circuit, Before I build this I would like to know which capacitor can I use for the Cc,Cin,Cout.Can I use a electrolytic capacitor, and if so how do you know which end will go towards the negative or it doesn't matter because the potential is all the same on both sides of the transistor?I know this is a dumb question but I still need help.I know, Ce can be an electrolytic capacitor and there will be no problem b/c you are tieing one lead to ground.Joe

author: TimFox
author_rank: Super Contributor
timestamp: March 18, 2020, 08:26:40 pm
content: Assuming that the source to the input and the load on the output are both at a DC voltage of zero (resistor to ground), then a polarized capacitor would have its positive lead to the transistor base (for Cin) or transistor collector (for Cout). The polarity of Cc will depend on the exact voltages at the collector of the first transistor and the base of the second transistor, which depends on the resistor values. Usually, the collector of the first transistor would be positive with respect to the base of the second transistor, but I could choose resistor values that would negate that statement. If the two-transistor circuit is driven from another transistor or other component with DC bias, you must check the voltages first. If you actually build this, the DC voltages will not change when adding the capacitors, so you an measure the actual voltages before you install Cin, Cc, and Cout.Whether you actually need an electrolytic capacitor, or can use a sensible value for a film capacitor (unpolarized) depends on the low-frequency behavior you demand from the circuit and the impedance levels (which also depend on the resistor values chosen).

author: Dellyjoe
author_rank: Regular Contributor
timestamp: March 18, 2020, 08:44:00 pm
content: Quote from: TimFox on March 18, 2020, 08:26:40 pmAssuming that the source to the input and the load on the output are both at a DC voltage of zero - what does this mean b/c for Vin i'm applying a small AC signal that is AC with 0VDC offset but that isn't DC voltage of zero?Usually, the collector of the first transistor would be positive with respect to the base of the second transistor, but I could choose resistor values that would negate that statementI designed the circuit to have the Vc to be Vcc/2 let's just call Vc=5v and then for the base of Q2 you have 1.7v, they are both positive so does that mean I shouldn't use an electrolytic capacitor? or b/c they are different potential then I put the negative side to the lower potential?I'm sorry I didn't really get what you are saying above.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 19, 2020, 07:15:21 am
content: Yes you can use electrolytes there.You can calculate polarity or get it from live circuit. Or use bipolar electrolytes.

author: TimFox
author_rank: Super Contributor
timestamp: March 19, 2020, 06:46:57 pm
content: Yes, if the input is an AC signal with DC level zero, and the first base is at a positive potential, then connect the negative end to the input and the positive end to the base. There will be almost no AC voltage across the capacitor (because it's a coupling capacitor), the AC on the base will follow the AC on the input, and the polarity only matters to make sure that the positive terminal is positive with respect to the negative terminal.I stated that the collector of the first transistor would normally be positive with respect to the second base, as in the example you gave in your second post, but I could not assume that since the voltages on the two transistors depend on your choice of resistors, and it would not be absurd to have the second base positive with respect to the first collector.