You will need to modify the circuit by adding a series capacitor (e.g. 1uF which can be polarised or non-polarised) at MONVID to drive capacitive load.
Image 2 is a 2nd order passive bandpass filter. This circuit is intended to filter noises from the signal, thus it has nothing to do with capacitive load.
You mentioned 4.5MHz, I believe you are refering to PAL system. If you include chromas and audio, you should be looking at 8MHz bandwidth.
Then you will need 16MHz or more.
I suggest you look at circuits that can operate around 25MHz.
If possible, check out IC and reference design from MAXIM and Intersil and Philips.
It's very unlikely and even more unwanted to have kohm output impedances for a buffer. I really wonder which circuits or devices you are referring to.BTW, unity-gain buffer (more commonly and correctly called, instead of source follower that refers more of MOS transistor configuration) offers medium output impedance around 150k to 200k Ohm. This is sufficient for capacitive load.
SkyHigh, second order with 4 reactive elements?...............
Image 2 is a 2nd order passive bandpass filter. This circuit is intended to filter noises from the signal, thus it has nothing to do with capacitive load.
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It's very unlikely and even more unwanted to have kohm output impedances for a buffer. I really wonder which circuits or devices you are referring to.
Video equipment is most using 75 ohm impedance matching, preferably at both source and load side. It's the only way to connect cables of certain length without causing double edges or echo images. The fact, that the discusses circuits are designed far from impedance matching súggests, that they are intended for internal short range signal connections only. Connecting a mostly reactive load to an amplifier, either discrete transistor or opamp circuits, brings up resonance peaking or even oscillations.
SkyHigh, second order with 4 reactive elements?
I'll be generating the signal coming in at COMVID. The circuits shown are all pre-existing in host computers and I have to have a single circuit that can drive them all equally well (or as well as I can get anyway.) I'm replacing an original device (TMS9918A VDP), and these are examples from computers that used that VDP.
So if I am feeding the signal in at Vin, what kind of load am I looking at?
NTSC actually, I was just being vague and over-stating the frequency for a little bit of a buffer. I will not be generating audio. Actually, I'm only going to generate 16 shades of gray, so no color either.
I found some examples on a few websites where composite video can be generated with just two resistors (which I have replicated), however you have to know the load resistance to select the resistor values. Since the load varies in my case, and I can't control that aspect, I started looking at using an opamp. Everything was going nicely until I read the datasheets about capacitive loads and such, for the video opamps.
I've been looking around, but as soon as you search on "video opamp", the frequency specs go to the 80MHz+ ranges. So you suggest I just find an opamp in the 25MHz range without specifically looking for one designed for video?
Thanks for the info, it is much appreciated!
Theoretically, 4 reactive elements should give you 4 Poles, but not always USUALLY 4th order.
Look at where the inductors are placed. That's right, in series.
1st node has a shunt C and series L, this is the 1st order that acts as low-pass filter.
2nd node has a series L and shunt R, but this node only servez as Zero, no Pole. In other words, only increasing Z at higher frequency compared to no or little voltage drop at DC.
The 2nd order kicks in with the series C after 2nd node. This will act as the high-pass filter.
With one 1st order high and one 1st order low-pass filter, you get a 2nd order band-pass.
Hi, Skyhigh.
May I give you the 4th order transfer function for the circuit under discussion?
Note that R2 is the input resistance of the BJT.
Numerator (nominator, sorry) : (C2 R1 R2)*s
Denumerator (Denominator):
R1 +(C2 R1 R2 + L2 + L1)*s
+(C1 L1 R1 + C2 L2 R2 + C2 L2 R1 + C2 L1 R2 + C2 L1 R1)* s^2
+ (C2 C1 L1 R1 R2 + C1 L1 L2)*s^3
+ (C2 C1 L1 L2 R2 + C2 C1 L1 L2 R1)*s^4
Regards
LvW
When you drive pure capacitive load (typical example is piezo-actuator) will required medium impedance at several k-Ohm.
I don't doubt, that "medium impedance" drivers, and particularly current sources play a role in driving capacitive loads. But they obviously aren't unity-gain buffers, and 150k to 200k is just an arbitrary number.BTW, unity-gain buffer (more commonly and correctly called, instead of source follower that refers more of MOS transistor configuration) offers medium output impedance around 150k to 200k Ohm. This is sufficient for capacitive load.
I don't see, how this statement and the succeeding considerations should be related to unity gain buffers. My comment was referring to exactly this statement:
I don't doubt, that "medium impedance" drivers, and particularly current sources play a role in driving capacitive loads. But they obviously aren't unity-gain buffers, and 150k to 200k is just an arbitrary number.
My impression so far is, that you are telling a lot, but it's neither exactly related to the problem nor well-founded.
Honestly, I won't post questions related to your contributions, if I don't doubt their technical correctness and relevance for the original problem as well. But I agree, that commenting your suggestions also doesn't help the original poster and I will stop it now, unless you'll shake electrical theory to the very foundations.Please relate to the topic to help the thread originator TQFP and stop disrupting this thread by asking your questions to please your queries that are not helping TQFP.
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To make it simple for you to understand, does 5th, 7th and 9th harmonics of a Fourier Series matter, when you have 1st, 2rd and 3rd harmonics?
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Sometimes I wonder if you actually ask someone for a change of $1.21 when he can give you $1.20
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