Automatic variable gain amplifier
An automatic variable gain amplifier is provided. The automatic variable gain amplifier automatically adjusts the amplification of a signal, and in one embodiment, an ion signal, based on the amplitude of the peaks of the signal. The automatic variable gain amplifier detects the peaks of the signal, compares them to a threshold value, and, based on this comparison, varies the amount by which the signal is amplified. The automatic variable gain amplifier produces a composite output waveform for an input waveform with an amplitude that may vary over a plurality of orders of magnitude.
1. An automatic two gain amplifier for use in conditioning an ion current signal sensed by an ion sensor in a combustion chamber of an engine, comprising:
an ion peak detector configured to receive the ion current signal from the ion sensor, the ion peak detector generating a an ion peak detector output approximating a peak magnitude of the ion current signal;
a comparator coupled to the ion peak detector and configured to compare the ion peak detector output with a threshold value, the comparator generating a first output when the ion peak detector output is greater than the threshold value and a second output when the ion peak detector output is less than the threshold value; and
an amplification circuit operatively coupled to the comparator and configured to receive the ion current signal, and the amplification circuit further configured to amplify the ion signal by a first amplification when the first output is generated by the comparator and to amplify the ion signal by a second amplification when the second output is generated by the comparator.
2. The amplifier of claim 1 , wherein the first amplification is less than the second amplification.
3. The amplifier of claim 1 , wherein the amplification circuit includes a control switch operative to change state when the comparator changes from the first output to the second output to change amplification from the first amplification to the second amplification.
4. The amplifier of claim 3 , wherein the amplification circuit includes an operational amplifier having a first feedback resistor configured to establish a gain thereof to provide the first amplification, and wherein the control switch operatively adds a second voltage dividing resistor to change the gain of the operational amplifier to provide the second amplification.
5. The amplifier of claim 4 , wherein the control switch includes a transistor coupled between the second resistor and ground.
6. The amplifier of claim 1 , wherein the first amplification is approximately unity.
7. The amplifier of claim 6 , wherein the second amplification is approximately 30.
8. The amplifier of claim 1 , wherein the second amplification is at least an order of magnitude greater than the first amplification.
9. The amplifier of claim 1 , wherein the threshold value is set to approximately five percent of a maximum first peak of the ion current signal.
10. The amplifier of claim 1 , wherein the comparator generates the second output when the ion current signal is below approximately seven microamperes.
11. A method of conditioning an ion current signal generated by an ion current sensor positioned within a combustion chamber, the ion current signal having a first peak and a second peak that may be twenty to one hundred times smaller than the first peak, comprising the steps of:
converting the ion current signal to a voltage signal;
tracking a peak magnitude of the voltage signal;
comparing the peak magnitude to a threshold; and
amplifying the voltage signal by a variable gain based on a relationship between the peak magnitude and the threshold.
12. The method of claim 11 , wherein the step of amplifying comprises the steps of:
amplifying the voltage signal by a first gain when the peak magnitude is greater than the threshold; and
amplifying the voltage signal by a second gain when the peak magnitude is less than the threshold.
13. The method of claim 12 , wherein the step of amplifying the voltage signal by the first gain when the peak magnitude is greater than the threshold comprises the step of amplifying the voltage signal by unity when the peak magnitude is greater than the threshold.
14. The method of claim 12 , wherein the step of amplifying the voltage signal by the second gain when the peak magnitude is less than the threshold comprises the step of amplifying the voltage signal by approximately thirty when the peak magnitude is less than the threshold.
15. The method of claim 11 , wherein the step of amplifying comprises the steps of:
amplifying the voltage signal by a first gain during at least the first peak of the ion current signal; and
amplifying the voltage signal by a second gain during at least the second peak of the ion current signal.
16. The method of claim 11 , wherein the step of amplifying comprises the steps of:
switching a gain from a high gain to a low gain when the step of comparing indicates that the peak magnitude is greater than the threshold; and
switching the gain from the low gain to the high gain when the step of comparing indicates that the peak magnitude is less than the threshold.
17. The method of claim 16 , wherein the step of switching the gain from the high gain to the low gain comprises the step of reducing the gain by at least an order of magnitude.
18. The method of claim 16 , wherein the step of switching the gain from the low gain to the high gain comprises the step of increasing the gain by at least an order of magnitude.
19. An ion current signal conditioning circuit for use in the control of an engine, the ion current signal having a first peak and a second peak, comprising:
an ion current sensor positioned within a combustion chamber of the engine;
a resistor coupled to the ion current sensor to generate an ion current voltage signal thereacross;
an ion peak detector configured to receive the ion current voltage signal, the ion peak detector generating a an ion peak detector output approximating a peak magnitude of the ion current voltage signal;
a comparator coupled to the ion peak detector and configured to compare the ion peak detector output with a threshold value, the comparator generating a first output when the ion peak detector output is greater than the threshold value and a second output when the ion peak detector output is less than the threshold value; and
an amplification circuit operatively coupled to the comparator and configured to receive the ion current signal, and the amplification circuit further configured to amplify the ion signal by a first amplification when the first output is generated by the comparator and to amplify the ion signal by a second amplification when the second output is generated by the comparator.
20. The ion current signal conditioning circuit of claim 19 , wherein the amplification circuit includes a control switch operative to change state when the comparator changes from the first output to the second output to change amplification from the first amplification to the second amplification, and wherein the amplification circuit includes an operational amplifier having a first feedback resistor and a second voltage dividing resistor configured to establish a gain thereof to provide the first amplification, and wherein the control switch operatively adds a second voltage dividing resistor to change the gain of the operational amplifier to provide the second amplification.