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Suppression Of The Series Capacitor In A Single-Ended Output Audio Application

IP.com Disclosure Number: IPCOM000008085D
Publication Date: 2002-May-16
Document File: 3 page(s) / 31K

Publishing Venue

The IP.com Prior Art Database

Abstract

In a single-ended audio application the audio amplifier is generally connected to the load via a series capacitor. The main reason for this is that the load (headphones for example) is connected on one side to the ground while the amplifier is generally mono-supply. There is then a difference between the DC voltage at the output of the amplifier (mid-supply approximately) and at the end of the load (ground). This results in very high current consumption, even in the absence of signal. The addition of a series capacitor allows absorption of this DC voltage difference without significant current consumption (refer to figure 1). The current across the load is then proportional to the input signal, and is null in the condition where there is no signal present at the input. However, the capacitor in series with the load forms a high pass filter which intorduces signal loss at low frequencies. If it is desired to transmit low frequencies (down to 50Hz with a 16 Ohms load for example) it is necessary to have a 220 micro Farad capacitor, which is very expensive. Therefor, it would be advantageous to develop a method and circuit for suppression of the series capacitor in a single-ended output audio application.

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Suppression Of The Series Capacitor In A Single-Ended Output Audio Application

In a single-ended audio application the audio amplifier is generally connected to the load via a series capacitor.  The main reason for this is that the load (headphones for example) is connected on one side to the ground while the amplifier is generally mono-supply.  There is then a difference between the DC voltage at the output of the amplifier (mid-supply approximately) and at the end of the load (ground).  This results in very high current consumption, even in the absence of signal. 

The addition of a series capacitor allows absorption of this DC voltage difference without significant current consumption (refer to figure 1). The current across the load is then proportional to the input signal, and is null in the condition where there is no signal present at the input. However, the capacitor in series with the load forms a high pass filter which intorduces signal loss at low frequencies. If it is desired to transmit low frequencies (down to 50Hz with a 16 Ohms load for example) it is necessary to have a 220 micro Farad capacitor, which is very expensive.

Therefor, it would be advantageous to develop a method and circuit for suppression of the series capacitor in a single-ended output audio application.

Figure 1

          The solution to the above prior art problems is to add an amplifier to generate a common mode +V/2 at the end of the load resistor, as can be seen in the new circuit of figure 2:

Figure 2

             T...