Function of Resistor in Clamper circuit

#12

Joined Nov 30, 2010
18,222
You call this a clamp. To me, it looks like a level shifter. It changes the center of the sine wave to a more positive level. If you look from a different point of view, it clamps the most negative excursion of the sine wave.

You see that the resistor is in parallel with the diode and battery. Move the resistor to the left and see that it allows current to flow in and out of the capacitor. Without the resistor, the diode would act as a rectifier like a voltage doubler circuit and the output would become a positive DC voltage. Therefore, the resistor keeps the circuit from being merely a rectifier. The resistor and capacitor must be the right amount compared to the frequency. If there is too much resistance, the circuit will tend to be a voltage doubling rectifier. With too little resistance the output will be attenuated.
 
You call this a clamp. To me, it looks like a level shifter.
Strictly speaking, that's precisely what a 'clamper' (not to be confused with a 'clamp') is (go figure:rolleyes:) -- FWIW I've long felt said bizarre nomenclature is cause of the considerable confusion of the terms 'Clamp', 'Clamper' and 'Snubber' (which being representative of distinct functions)... Wadda world! Wadda world!!!:(:rolleyes:

In case anyone cares, the following cursorily describes the functions from one another:

Clamp: Limits the maximum EMF across itself (e.g. MOVs, 'back-to-back' diodes, etc...)

Clamper/'clamping circuit':
Shifts the level of an AC waveform so as to 'set' either the maximum or minimum EMF

Snubber:
'Suppresses' and dissipates or 'recycles' transient energy


Best regards
HP
 
Last edited:

DickCappels

Joined Aug 21, 2008
7,562
Not intending to put too fine a point on it, back in the days when video meant an analog signal, we called that circuit a diode clamp or a DC restore circuit and it was used to establish the D.C. level of the black peaks of the signal, or in some cases the sync tips.

D.C.-wise the battery, diode, and resistor are all in series. The resistor provides the average bias current for the diode so that within a short time of turn-on the cathode of the diode is at V1-Vdiode on negative peaks of the signal. The resistor also assures that the output is at V1-Vdiode when no signal is present.
 

AnalogKid

Joined Aug 1, 2013
9,292
The master beat me to it. The schematic is a video "tip clamp". Note the polarity of the diode. By rule the negative peaks of the color subcarrier never can extend below the sync tip, so this clamp circuit is the first step in re-establishing the DC level of the video signal. In quality gear, it is followed by a "back porch clamp" that uses the trailing edge of the now-stabilized sync pulse to start a timed clamp (usually by a diode switch or transistor switch) in the space between the sync pulse and the video, to establish a DC level that is independent of changes in the sync pulse amplitude.

I miss real video.

ak
 

#12

Joined Nov 30, 2010
18,222
You can tell my history doesn't go back that far. When I was working on TVs I was pretty much a Tube Jockey practicing up to be a Beginner. I rarely got into the shop with the oscilloscopes so I could explore mysteries like, "How do you get a vacuum tube to set a DC level of one or two volts by working with 300 volts of supply and 20% resistor tolerance?" :confused:

I never did figure that out. I was on to the next job: precision analog design with the newfangled LM301 op-amp and some 1% resistors. :)
 

GopherT

Joined Nov 23, 2012
8,012
You can tell my history doesn't go back that far. When I was working on TVs I was pretty much a Tube Jockey practicing up to be a Beginner. I rarely got into the shop with the oscilloscopes so I could explore mysteries like, "How do you get a vacuum tube to set a DC level of one or two volts by working with 300 volts of supply and 20% resistor tolerance?" :confused:

I never did figure that out. I was on to the next job: precision analog design with the newfangled LM301 op-amp and some 1% resistors. :)
Do you need any of those LM301 or LM311H (in the fancy & trendy tin can package)? I know a guy who knows a guy.
 

Thread Starter

lynnfaiz

Joined Dec 16, 2012
29
You call this a clamp. To me, it looks like a level shifter. It changes the center of the sine wave to a more positive level. If you look from a different point of view, it clamps the most negative excursion of the sine wave.

You see that the resistor is in parallel with the diode and battery. Move the resistor to the left and see that it allows current to flow in and out of the capacitor. Without the resistor, the diode would act as a rectifier like a voltage doubler circuit and the output would become a positive DC voltage. Therefore, the resistor keeps the circuit from being merely a rectifier. The resistor and capacitor must be the right amount compared to the frequency. If there is too much resistance, the circuit will tend to be a voltage doubling rectifier. With too little resistance the output will be attenuated.
thanks for this answer. n thanks everyone too!
 
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