Block negative DC

[dan_thompson87]

Hi people,

As some of you may know I'm currently designing a compressor for my final year project at university. I currently have everything designed and tested on a breadboard. It all works perfectly however there is one quick fix I used that needs sorting before I can design the PCB.

The threshold control is simply an op amp in differential configuration. The threshold is controlled via a pot to give a fixed value which is then subtracted from the audio in. This leaves me with the amount of voltage that has crossed the threshold.

The problem I have is that as the op amp is dual DC powered - if the audio is below the level of the threshold I get a negative output voltage. This is undesirable as my design requires any audio below the threshold to give an output of 0V. The 'quick fix' around this that was used for testing was to put a diode on the output of the op amp. When the audio is below the threshold, the negative voltage is blocked by the diode. When it is above, the voltage drop across the diode is added back via another pot on a summing amplifier

The problem is that obviously the voltage drop across the diode varies for different input levels. Is there a circuit I can implement to make up for the varying voltage drop, or is there another way of blocking the negative voltage from the output?

Cheers

[AndrewT]

maybe this needs a precision rectifier. An opamp with diodes in the feedback that converts small signal AC to rectified AC ready to be peak sampled or otherwise modified.

[wakibaki]

Originally Posted by dan_thompson87

is there another way of blocking the negative voltage from the output?

Put the output through a unity-gain follower with a single supply rail.

No, thinking about it you can't exceed the rails on the input.

Andrew is right, a half-wave precision rectifier...

w

[dan_thompson87]

Originally Posted by AndrewT

maybe this needs a precision rectifier. An opamp with diodes in the feedback that converts small signal AC to rectified AC ready to be peak sampled or otherwise modified.

The signal begins with a precision rectifier that does as you've suggested. All of the signal being dealt with is positive up until the subtracting amplifier. Here's an example.

The threshold voltage is subtracted from the signal in:

If signal in = 2V and threshold = 1V then the output of the opamp is:

2 - 1 = 1V


If signal in = 1V and the threshold = 2V, then the output is:

1 - 2 = -1V

As you can see, any signal below the threshold will result in a negative voltage. However, in order for the compressor to work these negative voltages must somehow be blocked, or converted to 0V.

I hope this clears the question up!

[dan_thompson87]

Originally Posted by wakibaki

Put the output through a unity-gain follower with a single supply rail.

No, thinking about it you can't exceed the rails on the input.

Andrew is right, a half-wave precision rectifier...

w

Sorry, Andrew WAS right. I foolishly read his answer and thought of a full wave rectifier. I hadn't thought about using a half wave to block the negative! Such a simple solution too...

Thanks guys!