Patent Publication Number: US-2007096826-A1

Title: Methods and system of feedback in amplifier circuit

Description:
BACKGROUND  
      Emitter-follower circuits are used in the electronics industry to drive high current loads. The emitter-follower circuits (which may also be referred to as common-collector circuits or source-follower circuits) have the characteristic of having close-to-unity gain, which enables such circuits to interface between relatively low-powered control circuits (e.g., voltages produced by digital-to-analog converters) and large current loads.  
      Emitter-follower circuits are particularly susceptible to instability in the presence of capacitive loads connected to their output terminals (emitters) which instability leads to oscillation. The problem is exacerbated if there exists inductance in series with the base of the transistor. If the emitter-follower circuit becomes unstable and oscillates, improper operation of the circuit, excessive radio frequency emissions from the product, or even damage to the devices or the circuits may occur. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a detailed description of exemplary embodiments of the invention, reference will now be made to the accompanying drawings in which:  
       FIG. 1  shows an emitter-follower circuit in accordance with embodiments of the invention;  
       FIG. 2  shows a plurality of waveforms associated with an emitter-follower circuit in accordance with the embodiments of the invention;  
       FIG. 3  shows gain as a function of frequency for an illustrative emitter-follower circuit that does not utilize embodiments of the present invention;  
       FIG. 4  illustrates gain as a function of frequency for an emitter-follower circuit in accordance with the embodiments of the invention; and  
       FIG. 5  illustrates an emitter-follower circuit in accordance with alternative embodiments of the invention. 
    
    
     NOTATION AND NOMENCLATURE  
      Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, electronics companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function.  
      In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect connection via other devices and connections.  
      The term “collector” in this specification and the claims shall refer to the collector terminal of a bipolar junction transistor, and shall also refer to the drain terminal of a field-effect transistor (FET) and the family of related transistors. Thus, reference to a collector should not be construed as limiting the device to which the term refers to a bipolar junction transistor. Likewise, the term “base” shall refer to the base terminal of a bipolar junction transistor, and also shall refer to the gate terminal of a FET. Finally, the term “emitter” shall refer to the emitter terminal of a bipolar junction transistor, and also shall refer to the source terminal of a FET.  
      The terms “emitter-follower circuit,” “source-follower circuit,” or “common-collector circuit” in this specification and in the claims refers to amplifier circuits having unity voltage gain or less, and having greater than unity current gain.  
     DETAILED DESCRIPTION  
      The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure is limited to that embodiment.  
       FIG. 1  illustrates an emitter-follower circuit  100  in accordance with embodiments of the invention. In particular,  FIG. 1  illustrates a signal or control voltage source (V s )  10  coupled to the base  38  of a transistor  12 . The control voltage source  10  may be any suitable source for providing a signal to the emitter-follower circuit  100  (e.g., an analog signal created by a digital-to-analog (DA) converter).  FIG. 1  shows transistor  12  to be a bipolar junction transistor, and in particular an n-p-n bipolar transistor; however, using a bipolar junction transistor is only illustrative, and the systems and related methods described in this specification are equally applicable to other transistors, such as field-effect transistors (FETS).  
      In some embodiments, the control voltage source  10  couples to the base  38  by way of a base resistor (R B )  14  and optionally a base inductor (L B )  16 . The base resistor  14  and base inductor  16  may each be individual circuit components, or either or both of these may be parasitic (i.e., caused by degradation and/or imperfection of connections within the transistor). The emitter  18  of the transistor  12  couples to a load  20 , which load may take many forms (e.g., a motor optionally coupled to a fan, a relay, a solenoid, or heaters). For purposes of illustration, load  20  is shown to have a load resistance (R L )  22  and a load capacitance (C L )  24 , although any circuit element may be present in the load.  
      Emitter-follower circuits  100  in accordance with the embodiments of the invention exhibit the characteristic of having slightly less than unity gain.  FIG. 2  shows a graph as a function of time of the voltage of an illustrative signal  30  provided by the control voltage source  10  to the transistor  12 .  FIG. 2  also shows an illustrative output voltage of the emitter-follower circuit  100  as a function of time co-plotted with the line  30 . In particular, line  32  (dash-dot-dash) shows the output voltage (V O ) signal which may be seen at the output terminal (emitter) of the emitter-follower circuit  100  given the control signal  30 . As illustrated in  FIG. 2 , the voltage of the output voltage signal  32  is slightly less than the voltage of the signal provided by the control voltage source  10 . While there is little if any voltage gain, the advantage of an emitter-follower circuit is its large current gain. Thus, while the voltage source  10  may only be capable of sourcing a few tens of milli-amps, the emitter-follower circuit  100  may be capable of sourcing current sufficient to drive high current devices (e.g., motors, relays, and solenoids).  
      Emitter-follower circuits are susceptible to instability based on the interplay of base inductance  16  with load capacitance  24 . Consider for purposes of explanation the emitter-follower circuit  100  without the feedback circuit  34  (the feedback circuit  34  is discussed below). Stated otherwise, consider a situation where the supply voltage (V CC ) couples directly to the collector  36 , and there is no external connection between the collector  36  and the base  38 . In such a situation, and in the presence of base inductance  16  and load capacitance  24 , the voltage gain as a function of frequency may be as illustrated in  FIG. 3 . The asymptotic response of the voltage gain at frequency F 1  is illustrative of a frequency at which the emitter-follower circuit is unstable. A circuit having an instability point such as this tends to oscillate, even if the signal provided by the control voltage source  10  is not sinusoidal in nature.  
      Returning again to  FIG. 1 , the inventor of the present specification has found that the instability in a emitter-follower circuit can be reduced by providing a negative or degenerative rate feedback to the base  38  of the transistor  12 . More particularly, the inventor of the present specification has found that providing a feedback whose amplitude increases with frequency (rate feedback) based on the current in the collector  36  produces a decreasing emitter-follower circuit  100  gain as a function of increasing frequency. Stated otherwise, various embodiments provide a feedback to the base of the transistor of the emitter-follower circuit with the feedback proportional to both the amplitude of the collector current and frequency of the collector current. Inasmuch as the current in the collector  36  (as well as the voltage) is approximately 180 degrees out of phase with respect to the control voltage source  10 , coupling a feedback proportional to the collector current to the base  38  is a degenerative or negative feedback which tends to cancel a portion of the signal of the control voltage source  10 , thus reducing the effective gain of the emitter-follower circuit  100 .  
      At least some embodiments provide the negative rate feedback by way of a feedback circuit  34 . In accordance with these embodiments, feedback circuit  34  comprises a collector resistor (R C )  40  (coupled between the supply voltage (V CC ) and the collector  36 ), and a feedback capacitor (C F )  42 . The resistance of collector resistor  40  is relatively small (guidelines for selecting values of the collector resistor  40 , feedback capacitor  42  and base resistor  14  are discussed below) and thus current in the collector  36  creates a feedback voltage (V FB ) at the collector  36  which is proportional to the current in the collector  36 .  
       FIG. 2  illustrates the feedback voltage  52  at the collector  36  with the collector resistor in place and the control voltage source  10  providing the illustrative control signal  30 . In particular, feedback voltage  52  created by the presence of the collector resistor has the same frequency as the control signal  30  from the control voltage source  10 . The feedback voltage, however, is 180 degrees out of phase with respect to the control signal  30  from the control voltage source, and the peak-to-peak amplitude of similar features, (e.g., between the peak  54  and peak  56  of signal  30 ) are smaller. In these embodiments, the feedback voltage created by the presence of the collector resistor  40  couples to the base  38  through the feedback capacitor  42 , which, from an impedance standpoint, decreases in impedance as the frequency increases.  
       FIG. 4  shows voltage gain as a function of frequency for an emitter-follower circuit  100  having components and values similar to those whose gain is illustrated by  FIG. 3 , but utilizing a negative rate feedback circuit  34  in accordance with the embodiments of the invention. In particular,  FIG. 4  shows that because of the feedback proportional to both the amplitude of the collector current and frequency of the collector current, the gain falls off with increasing frequency such that at the frequency where the circuit previously went unstable, F 1 , no instability is present.  
      The discussion now turns to selecting values of various components for proper operation of the negative rate feedback in accordance with the embodiments of the invention. It is first noted that the base resistor  14  is not required, although there will be in most instances parasitic resistance present (and this could be considered some or all the base resistor). A base resistor  14  may be used in situations where the impedance of the voltage source  10  is lower than the inherent base resistance of the transistor  12  at nominal operating current. Thus, the resistance of the base resistor is chosen such that the source impedance (Z S ) considered with resistance of the base resistor  14  (Z S+ R B ) is greater than or equal to the inherent base resistance of the transistor, which may be referred to in product specifications for bipolar junction transistors as re.  
      With respect to the selection of the collector resistor  40 , increasing collector resistance decreases emitter-follower circuit dynamic range, and thus lower resistance values are better from a peak-to-peak output voltage perspective. However, the greater the resistance of the collector resistor  40 , the greater the amplitude of the feedback voltage. In accordance with the embodiments of the invention, the emitter-follower circuit should be stable when the source impedance Z S  summed with the resistance of the base R B  (if present) is greater than the collector resistance R C , and the sum of the source impedance Z S  and the resistance of the base R B  multiplied by the capacitance of the feedback capacitor C F  is greater than or equal to approximately four times the product of the collector resistance R C  and the load capacitance C L . More particularly with respect to the first relationship, the sum of the source impedance Z S  and the resistance of the base R B  should be approximately ten times or more than resistance of the collector resistor.  
      In more mathematical terms then, when: 
 
(Z S +R B )&gt;&gt;R C   (1) 
 
 where Z S  is the source impedance, R B  is the base resistance (if any), and R C  is the collector resistance, then stability is conservatively ensured when: 
 
 C   F ·( Z   S   +R   B )≧4 ·R   C   ·C   L   (2) 
 
 where Z S , R B , and R C  are as above, C F  is the feedback capacitance and C L  is the load capacitance. 
 
      The various embodiments discussed to this point use a bipolar junction transistor and a feedback circuit coupled between the collector and the base.  FIG. 5  illustrates an emitter-follower circuit  102  in accordance with alternative embodiments of the invention. In particular,  FIG. 5  illustrates that in alternative embodiments of the invention, rather than using a bipolar junction transistor, a FET  60  may be used. In such a situation, the emitter may be equivalently referred to as the “source,” the collector equivalently referred to as the “drain,” and the base equivalently referred to as the “gate.” Moreover,  FIG. 5  illustrates an alternative feedback circuit  62  that may be used with either a bipolar junction transistor or a FET, and this alternative feedback circuit  62  couples between the emitter (source)  64  and the base (gate)  66 . Because the voltage at the emitter (source)  64  in an emitter-follower circuit is in phase with the signal produced by the control voltage source  10 , in order to provide negative feedback the illustrative feedback circuit  62  preferably generates an output signal that is an inverted version of the output voltage (V O ) prior to coupling the output signal to the base (gate)  66 . The illustrative embodiments of  FIG. 5  show such a circuit in the form of an operational amplifier configured as an inverting amplifier. However, the operational amplifier is illustrative of any inverting amplifier circuit (e.g., inverting amplifier made from individual components such as resistors and transistors). The gain of the amplifier in the feedback circuit  62  in accordance with these alternative embodiments is preferably selected to mimic a voltage that may be generated by coupling a collector resistor between the power rail source and the collector (drain)  68  (see, e.g., feedback voltage  52  of  FIG. 2 ). Thus, the values for the resistor (R 1 )  70  and the feedback resistor (R 2 )  72  in feedback circuit  42  should thus be chosen to mimic a feedback voltage signal as discussed with respect to the feedback circuit  34  of  FIG. 1 . Feedback circuit  42  further comprises a feedback capacitor  72  that couples the output signal of the illustrative operational amplifier  74  to the base (gate)  66 .  
      Thus,  FIG. 5  illustrates that the negative rate feedback related to the current in the collector (drain) need not be generated on the collector (drain) side of the amplifier, and instead may be generated proportional to the output voltage as illustrated in  FIG. 5  using an inverting amplifier. In yet still further embodiments, the current within the collector (drain) may be sensed by other methods such as Hall effect devices or current transformers.  
      The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the embodiments of illustrative  FIG. 1  show the use of an external feedback capacitor  42  as a part of the feedback circuit  34 . In alternative embodiments of the invention the transistor  12  and/or FET  60  may be selected such as the inherent parasitic capacitance between the collector (drain) and the base (gate) is sufficient to fulfill the purposes of the feedback capacitor  42 , and in these cases a separate feedback capacitor  42  may be omitted.