Abstract:
An apparatus and method for improving the speed and accuracy of a feedback amplifier when the primary feedback around the amplifier configuration has been interrupted is provided. An additional feedback loop added to the input stage of the amplifier maintains the circuit response by maintaining the feedback and preventing the saturation of the circuit components.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to feedback amplifiers. More particularly, it relates to a method of improving the speed and accuracy of a circuit utilizing a feedback amplifier configuration, such as, for example, a peak detector. 
     2. The Prior Art 
     In a basic peak detector circuit, as the input voltage increases, the feedback configuration of the circuit causes the circuit to go into a voltage follower mode. That is, the output voltage is equal to the input voltage. When the input voltage subsequently decreases, the feedback is interrupted, thereby causing the circuit elements to leave their normal operation mode. When the feedback is then restored, recovery of the circuit can take too long to allow for accurate tracking of the input signals. 
     SUMMARY OF THE INVENTION 
     The present invention provides an additional feedback loop added to the basic feedback configuration of a peak detector to enable faster and more accurate recovery after the primary feedback around the amplifier configuration has been interrupted. 
     It is therefore an object of the present invention to provide an apparatus and method for improving the speed and accuracy of a circuit utilizing a feedback amplifier configuration. 
     It is another object of the invention to provide an apparatus and method for increasing the recovery rate of the feedback loop in a circuit utilizing a feedback amplifier configuration. 
     Yet another object of the invention is to provide an apparatus and method for increasing the recovery rate of the feedback loop in a circuit utilizing a feedback amplifier configuration that operates efficiently and reliably. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose an embodiment of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. 
     In the drawing, wherein similar reference characters denote similar elements throughout the several views: 
     FIG. 1 is a schematic circuit diagram of a peak detector of the prior art; 
     FIG. 2 is a graphical representation of the transient response of the peak detector circuit of FIG. 1; 
     FIG. 3 is a schematic circuit diagram of a peak detector according to the invention; and 
     FIG. 4 is a graphical representation of the transient response of the peak detector circuit of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a peak detector circuit of the prior art. The peak detector circuit consists of an input stage Q1, Q2 followed by a second stage Q5, a diode D2, and capacitor C2. As the input voltage V in  increases, the feedback from the output to the base of Q2 forces the circuit into a voltage follower mode (i.e., the output voltage is equal to the input voltage). If the input voltage V in  decreases, the voltage on capacitor C2 remains constant and the feedback to the base of Q2 is interrupted. Diode D2 is biased to only allow current to flow from current source I2 to capacitor C2 and not vice versa. 
     When the input voltage decreases, the difference between the input voltage V in  and the output voltage V out  is negative, thereby forcing the collector voltage (V cQ5 ) of the second stage input transistor Q5 to go negative and Q5 into saturation. Therefore, when the input voltage V in  subsequently increases beyond the voltage stored at the output on capacitor C2, the collector voltage of Q5 (V cQ5 ) is brought back up. As a result of the saturation of Q5, the charge built up on the base of Q5 causes this response to be delayed. 
     One well-known method of preventing saturation is to clamp transistor Q5 with a Schottky diode D1. When the input voltage V in  increases beyond the stored output voltage, the collector voltage of Q5 starts to rise immediately at a rate set by the current source I1 and Miller capacitor C1. As a result of this slewing effect, there still remains a delay before V out  follows V in . 
     FIG. 2 shows the transient response of the peak detector of FIG. 1. As shown, the peak of the input voltage V in  has already passed when the output V out  is slewed back up. The implication is that for fast changing signals relative to the slew rate of the peak detector, this delay can cause accuracy errors. In addition, if the circuit consists of more than the two stages shown in FIG. 1, more than one Schottky diode may be needed to prevent saturation of other transistors. This is undesirable due to the increased size of the die that would be required to make the circuit. 
     The circuit of FIG. 3 is similar to that of the prior art circuit of FIG. 1, except that the Schottky diode D1 has been removed and an additional input transistor Q2b has been added. Transistor Q2b has its emitter and collector coupled to the emitter and collector of transistor Q2a, respectively, while the base of Q2b is connected to the collector of Q5. As described earlier, if the input voltage decreases below the value that is stored on output capacitor C2, the feedback is interrupted and diode D2 becomes reverse biased. As a result of the voltage decrease, the collector voltage of Q5 (V cQ5 ) decreases, but now only to one diode voltage below the output voltage. At this point, and before any further decrease in V cQ5 , the feedback is restored through the additional input transistor Q2b. As a result of this operation, the remaining circuit elements to the left of diode D2 remain properly biased and particularly not in saturation. 
     When the input voltage is subsequently increased beyond the voltage stored on capacitor C2, the circuit restores the feedback through input transistor Q2a. Therefore, the collector voltage (V cQ5 ) of Q5 now only has to slew over one diode voltage, and the response of the circuit is much faster. 
     FIG. 4 shows the transient response of the circuit of FIG. 3 according to the invention. Since the feedback response is much faster, the output V out  can now accurately track the input voltage V in . Under these circumstances, the collector voltage of Q5 (V cQ5 ) is not forced negative, and the response time is significantly increased. 
     Although the additional feedback loop of the circuit shown in FIG. 3 consists of bipolar pnp transistors, the same approach can also be applied to circuits consisting of MOS transistors, In addition, both p-type and n-type devices can be used in the primary and additional feedback loop. 
     While one embodiment of the present invention has been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims.