Abstract:
A method and apparatus are directed to emulating an emitter follower with a small PNP transistor that is arranged in a PNP multiplier configuration. The PNP multiplier includes a PNP emitter follower and a current mirror. The PNP follower is coupled between the input and the output. A current mirror is coupled to the collector of the PNP follower such that mirror produces a current that is a scaled version of the collector current from the PNP follower. The current mirror is arranged to scale the PNP collector current by a factor of N. The effective output current from the PNP multiplier circuit corresponds to β·I IN ·(N+1), where β corresponds to the large signal forward gain of the PNP follower. By multiplying the output current by a scaling factor, the effective forward gain of the PNP transistor is increased while utilizing a small geometry PNP device.

Description:
FIELD OF THE INVENTION 
     The present invention relates to emitter followers. In particular, the present invention relates to an apparatus and method that has a voltage gain and current gain that are comparable to a PNP emitter follower circuit that employs a PNP transistor with a large die area. 
     BACKGROUND OF THE INVENTION 
     An emitter follower is a single transistor circuit that is arranged in a common-collector configuration. The transistor in the emitter follower can be an NPN transistor or a PNP transistor. An emitter follower has a voltage gain from the base of the transistor to the emitter that is close to one. The emitter follower is often used to provide a high current output since the transistor has a high current gain. Additionally, the emitter follower has a high input impedance and a low output impedance. An emitter follower is useful for isolating or buffering a high-resistance source such that a low-resistance load does not excessively load down the source. 
     SUMMARY OF THE INVENTION 
     A method and apparatus are directed to emulating an emitter follower with a small PNP transistor that is arranged in a PNP multiplier configuration. The PNP multiplier includes a PNP emitter follower and a current mirror. The PNP follower is coupled between the input and the output. A current mirror is coupled to the collector of the PNP follower such that mirror produces a current that is a scaled version of the collector current from the PNP follower. The current mirror is arranged to scale the PNP collector current by a factor of N. The effective output current from the PNP multiplier circuit corresponds to β·I IN ·(N+1), where β corresponds to the large signal forward gain of the PNP follower. By multiplying the output current by a scaling factor, the effective forward gain of the PNP transistor is increased while utilizing a small geometry PNP device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating an exemplary operating environment for a PNP multiplier circuit; 
     FIG. 2 is a schematic diagram of a first example PNP multiplier; 
     FIG. 3 is a small signal model of the PNP multiplier illustrated in FIG. 2; 
     FIG. 4 is a simplified small signal model of the PNP multiplier illustrated in FIG. 2; 
     FIG. 5 is a schematic diagram of another PNP multiplier; 
     FIG. 6 is a schematic diagram of still another PNP multiplier; 
     FIG. 7 is a schematic diagram of yet another PNP multiplier; and 
     FIG. 8 is a schematic diagram of still yet another PNP multiplier, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the specification, and in the claims, the term “connected” means a direct electrical connection between the things that are connected, without any intermediary devices. The term “coupled” means either a direct electrical connection between the things that are connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means one or more passive and/or active components that are arranged to cooperate with one another to provide a desired function. The term “signal” means at least one current signal, voltage signal, electromagnetic wave signal, or data signal. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on”. 
     FIG. 1 is a schematic diagram illustrating an operating environment ( 100 ) for the present invention. An exemplary environment ( 100 ) of the present invention includes a driver circuit ( 102 ), a PNP multiplier ( 104 ), voltage source VCC, and an output port (VOUT). Driver circuit  102  has an output that is coupled to an input of PNP multiplier  102 . PNP multiplier  102  is also coupled to voltage source VCC and output port VOUT. PNP multiplier  102  operates as an emitter follower circuit. 
     The present invention utilizes an arrangement of transistors that operate similar to an emitter follower circuit. The sizes of the transistor devices used to achieve the emitter follower are small (i.e., die area) while providing an appropriately high current gain. The apparatus includes a small PNP emitter follower that is coupled to a current mirror. The ratio of the current mirror determines the current gain of the apparatus. The apparatus gives comparable performance to a large PNP transistor, without the large die area required by a conventional emitter follower. 
     FIG. 2 shows a PNP multiplier circuit ( 200 ) that is in accordance with the present invention. As shown in the FIGURE, PNP multiplier  200  includes a PNP transistor (Q 21 ) and two NPN transistors (Q 22 -Q 23 ). Transistor Q 21  has a base that is coupled to node N 20 , an emitter that is coupled to node N 23 , and a collector that is coupled to node N 21 . Transistor Q 22  is a diode-connected device that has a base and a collector that are coupled to node N 21 , and an emitter that is coupled to node N 22 . Transistor Q 23  has a base that is coupled to node N 21 , an emitter that is coupled to node N 22 , and a collector that is coupled to node N 23 . Node N 22  is connected to ground terminal (GND). 
     Transistor Q 21  is a PNP emitter follower that has an emitter that provides an output voltage (V OUT ) in response to the input voltage (V IN ) that is supplied to the base. An input current (I IN ) is drawn from the base of transistor Q 21  as indicated on the figure. Transistor Q 21  has an associated forward gain (β). The collector current of transistor Q 21  is determined by β·I IN . Transistors Q 22  and Q 23  are configured to operate as a current mirror with a 1:N ratio. Transistor Q 23  has a collector current that is determined by β·N·I IN . The output current (I OUT ) corresp-onds to the sum of the collector current for transistor Q 23  and the emitter current for transistor Q 21 . Neglecting base currents for transistors Q 22  and Q 23 , the collector current (I OUT ) of transistors Q 21  and Q 23  are determined by β·I IN ·(N+1). The total effective current gain for the PNP multiplier ( 200 ) is thus given by β·(N+1). 
     The PNP multiplier ( 200 ) is arranged to act as the equivalent of a large PNP device with a relatively high current gain characteristic. PNP multiplier  200  provides a current gain of β·(N+1) at low frequencies as will be discussed in further detail. The value of N is chosen according to the desired current gain for the PNP multiplier circuit ( 200 ). By scaling the ratio of the current mirror, the current gain can be scaled by the factor N. A small PNP transistor (Q 21 ) may be used such that die area is conserved. The gain of the transistor Q 21  is increased by the multiplication factor (N) created by the current mirror. 
     Although the diagram shows a 1:N current mirror, other embodiments are possible. For example, a current mirror could be coupled to transistor Q 21  via a cascode arrangement. This would improve the performance of the circuit at higher voltages. At higher voltages, the Early effect may cause the current at the collector of transistor Q 23  to be significantly less than β·N·I IN . The cascode arrangement prevents the mismatch in the current scaling that would otherwise result from the Early effect. 
     FIG. 3 is a schematic diagram of a small signal model ( 300 ) that corresponds to the PNP multiplier ( 200 ) shown in FIG.  2 . The small signal model ( 300 ) includes equivalent circuits for transistors Q 21 , Q 22 , and Q 23 . The equivalent circuit for transistor Q 21  includes two resistors (r 11 , ro 1 ), a controlled current source I 31 , and two capacitors (C 11 , C 12 ). The equivalent circuit for transistor Q 22  includes a resistor (r 21 ) and a capacitor (C 21 ). The equivalent circuit for transistor Q 23  includes a capacitor (C 32 ) a controlled current source ( 133 ), and a resistor (ro 3 ). 
     Resistor r 11  is coupled between node N 20  and node N 23 . Capacitor C 11  is coupled between node N 20  and node N 23 . Controlled current source  131  is coupled between node N 23  and node N 21 . Resistor ro 1  is coupled between node N 23  and node N 21 . Capacitor C 12  is coupled between node N 20  and node N 21 . Resistor r 21  is coupled between node N 21  and node N 22 . Capacitor C 21  is coupled between node N 21  and node N 22 . Capacitor C 32  is coupled between node N 21  and node N 23 . Controlled current source  133  is coupled between node N 22  and node N 23 . Resistor ro 3  is coupled between node N 22  and node N 23 . 
     An output voltage appears at node N 23  in response to an input voltage (V IN ) that is applied to node N 20  during operation. An intermediary voltage (V C ) is produced at node N 21 . Controlled current source  131  provides a current that is determined by gm 1 ·(V OUT −V IN ), where gm1 corresponds to the transconductance of transistor Q 21 . Resistor r 21  has a resistance that corresponds to 1/gm 2 , where gm 2  corresponds to the transconductance of transistor Q 22 . Controlled current source  133  provides a current that corresponds to N·gm3·V C . 
     FIG. 4 shows a schematic diagram ( 400 ) that corresponds to a simplified small signal model  400  for small signal model  300 . Simplified small signal model  400  includes resistor r 11 , capacitor C 11 , capacitor C 12 , controlled current source  140 , and resistor ro 4 . Resistor r 11  is coupled between node N 20  and node N 23 . Capacitor C 11  is coupled between node N 20  and node N 23 . Capacitor C 12  is coupled between node N 22  and node N 20 . Current source  140  is coupled between node N 23  and node N 22 . Resistor ro 4  is coupled between node N 23  and node N 22 . 
     Resistor ro 4  has a value that corresponds to the parallel combination of ro 1  and ro 3 . Controlled current source  140  provides a current that is determined by:          I40   =         N   ·   g                     m   ·     (       V   OUT     -     V     I                 N         )               1   gm     ·   s   ·     (       (     2   ·     C   32       )     +     C   21       )       +   1         ,                          
     where gm=gm 1 =gm 2 =gm 3 . 
     The above illustrated small signal model results in a pole at a frequency that corresponds to        gm       (     2   ·     C   32       )     +     C   21                              
     Since capacitors C 21  and C 32  are normally very small, the pole is at a high frequency. The pole will cause PNP multiplier  200  to act differently than a conventional PNP circuit at high frequencies. Specifically, PNP multiplier  200  will have a lower current gain and a lower phase margin than a conventional PNP emitter follower at high frequencies. However, at lower frequencies, PNP multiplier  200  will behave the same as a PNP emitter follower with a large die area. 
     As previously stated, the ratio (N) that is provided by the current mirror as determined by transistors Q 22  and Q 23 . The ratio may be achieved by scaling the area of the transistors, by providing a parallel combination of transistors, or any combination of area scaled and paralleled transistors. For example, a scaling factor (N) of twenty can be achieved by arranging twenty-one identically sized transistors. In this instance, twenty of the transistors are connected in parallel to provide Q 23 , while one transistor is configured as diode connected transistor Q 22 . 
     Although the previous illustrations have a simple two transistor current mirror, other arrangements may replace transistors Q 22  and Q 23  and provide the same functionality in the PNP multiplier. FIGS. 5-8 illustrate exemplary alternative current mirrors. 
     FIG. 5 illustrates a PNP multiplier that employs an emitter degenerated current mirror. The arrangement is substantially similar to the current mirror described in FIG. 2 with the addition of resistors R 52  and R 53 , which are coupled between ground (GND) and the emitters of transistors Q 22  and Q 23  respectively. The operation of the current mirror yields a similar result to that previously provided, while also increasing the output impedance of the current mirror. 
     FIG. 6 illustrates a PNP multiplier that employs a Widlar current mirror. The arrangement is substantially similar to the current mirror described in FIG. 2 with the addition of a resistor R 63 , which are coupled between ground (GND) and the emitter of transistor Q 23 . The operation of the current mirror yields a similar result to that previously provided, while also increasing the output impedance of the current mirror. 
     FIG. 7 illustrates a PNP multiplier that employs a Wilson current mirror. The arrangement is substantially similar to the current mirror described in FIG. 2 with the addition of transistor Q 73 . However, in FIG. 7, transistor Q 22  is not diode connected, while transistor Q 23  is diode connected (collector and base coupled together at node N 73 ). Transistor Q 73  is an NPN transistor that is arranged as a cascode transistor that is series connected between the output at node N 23  and the collector of Q 23  at node N 24 . Transistor Q 73  has a base that is coupled to node N 21 , a collector that is connected to node N 23 , and an emitter that is connected to node N 73 . The operation of the current mirror yields a similar result to that previously provided, while also increasing the output impedance of the current mirror. 
     FIG. 8 illustrates a PNP multiplier that employs a cascode current mirror. The arrangement is substantially similar to the current mirror described in FIG. 2 with the addition of transistors Q 82  and Q 83 . Transistor Q 82  is a diode connected NPN device that is connected in series between transistors Q 21  and Q 22 . Transistor Q 83  is an NPN device that is connected in series between the output and transistor Q 23 . The collector and base of transistor Q 82  are connected to the collector of transistor Q 21  at node N 82 , while the emitter of transistor Q 82  is connected to node N 21 . The collector of transistor Q 83  is connected to the output at node N 23 . The base of transistor Q 83  is connected to node N 82 , while the emitter is connected to the collector of transistor Q 23  at node N 83 . The operation of the current mirror yields a similar result to that previously provided, while also increasing the output impedance of the current mirror. 
     Although the preceding description describes various embodiments of the system, the invention is not limited to such embodiments, but rather covers all modifications, alternatives, and equivalents that fall within the spirit and scope of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.