# Patent

## Title
Push-pull buffer/amplifier

## Abstract

A differential circuit comprises first, second, third and fourth devices that have first, second, third and fourth control terminals, respectively. The second and fourth devices are arranged in series with the first and third devices, respectively. A first output communicates with the first device and the second device. A second output communicates with the third device and the fourth device. First and second inputs communicate with the first and fourth control terminals, respectively. The first, second, third and fourth devices are the same type of devices. The third and second control terminals follow the first and fourth control terminals. When the first device pushes, the third device pulls. When the first device pulls, the third device pushes. When the fourth device pushes, the second device pulls. When the fourth device pulls, the second device pushes. The second and third devices and the first and fourth devices are matched.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. patent application Ser. No. 10/744,184 filed Dec. 23, 2003, now U.S. Pat. No. 6,958,650 which claims the benefit of U.S. Provisional Application No. 60/470,603 filed May 14, 2003, which are both hereby incorporated herein by reference in their entirety.


FIELD OF THE INVENTION
The present invention relates to buffer and amplifier circuits, and more particularly to buffer and amplifier circuits that drive low impedance capacitive loads at high frequencies.
BACKGROUND OF THE INVENTION
Buffer and amplifier circuits include emitter follower circuits of the type illustrated in FIG. 1. The emitter follower is typically characterized by a high input impedance and a low output impedance. Therefore, it is useful as an isolation or buffer amplifier (e.g., a unity gain buffer) for connecting a high impedance source to a low impedance load.
With further reference to FIG. 1, when control terminal V+ is pulled sufficiently high, transistor Q1 is biased on to provide a corresponding voltage at positive output terminal Vout+. When control terminal V+ is pulled low, transistor Q1 is biased off and transistor Q3 draws current from a load (not shown) via the positive output terminal Vout+. Similarly, when control terminal V− is pulled high, transistor Q2 is biased on to provide a corresponding voltage at negative output terminal Vout−. When control terminal V− is pulled low, transistor Q2 is biased off and transistor Q4 draws a current from the load via the negative output terminal Vout−.
One disadvantage of the circuit shown in FIG. 1 is that it consumes excessive power. Specifically, transistors Q3 and Q4 are biased on by the current source 102 and transistor Q5. Therefore, when the control terminal V+ is pulled high, transistor Q1 provides the current drawn by transistor Q3 in addition to providing current to the load via positive output terminal Vout+. Likewise, when the control terminal V− is pulled high, transistor Q2 provides the current drawn by transistor Q4 in addition to providing current to the load via the negative output terminal Vout−. Thus, the current drawn by transistors Q3 and Q4 when transistors Q1 and Q2, respectively, are on is wasted.
For high frequency applications involving capacitive (including partly capacitive) loads, the overall impedance of the capacitive load is typically low. Therefore, the transconductance of the transistors must be increased in order to drive the low impedance. Additionally, the bias current of the circuit shown in FIG. 1 must be sufficiently large such that the output stage is not slew limited. As frequency is increased, emitter followers of the type shown in FIG. 1 require additional current in order to drive low impedance loads.
FIG. 2 illustrates a known common-emitter differential amplifier. When control terminal V+ is pulled low, transistor Q6 is biased off such that current is provided from voltage supply Vcc and through a resistor R1 to a load via negative output terminal Vout−. Conversely, when control terminal V− is pulled low, transistor Q7 is biased off such that current is provided from the voltage supply Vcc and through a resistor R2 to the load via positive output terminal Vout+.
In the case of capacitive loads, voltage supply Vcc is essentially coupled to an RC circuit. The load represents the capacitance C of the RC circuit. Thus, as the value R of resistors R1 and R2 is increased, the bandwidth of the differential amplifier decreases. Specifically, the bandwidth of the amplifier is given by: 



  
    f
    BW
  
  =
  
    1
    
      2
      ⁢
      
        π
        ⁢
        RC
      
    
  




For high frequency applications, it is often desirable to have fBW>>fopt, where fopt is the operating frequency. If R is too small, however, excessive current will be conducted through transistors Q6 and Q7. This is because the amplitude of the output is given by ibiasR. Assuming transistors Q6 and Q7 are fully switched such that the current is steered to any one side during operation, a relatively large current is required to have sufficient amplitude during high frequency operation.
Referring now to FIG. 3, a class B push-pull amplifier is shown. When control terminal Vin is pulled high, transistor Q8 is biased on and transistor Q9 is biased off, resulting in transistor Q8 “pushing” current to a load via output terminal Vout. Conversely, when control terminal Vin is pulled low, transistor Q8 is biased off and transistor Q9 is biased on, resulting in transistor Q9 “pulling” current from the load via the output terminal Vout.
The buffer of FIG. 3 is generally efficient in terms of current consumption as compared to the buffer/amplifiers of FIGS. 1 and 2. However, transistor Q9 is typically a PNP bipolar junction transistor or a p-type MOS transistor. Because PNP bipolar junction transistors and p-type MOS transistors have less transconductance than their n-type counterparts, the buffer of FIG. 3 may either significantly load a previous stage to which the buffer is coupled, or the p-type transistor Q9 may be so weak that output waveforms are distorted. Further, p-type devices typically have low gm values and therefore are generally not well suited for high frequency applications.
SUMMARY OF THE INVENTION
A differential circuit comprises first, second, third and fourth devices that have first, second, third and fourth control terminals, respectively. The second and fourth devices are arranged in series with the first and third devices, respectively. A first output communicates with the first and second devices. A second output communicates with the third and fourth devices. First and second inputs communicate with the first and fourth control terminals, respectively. The first, second, third and fourth devices are the same type of devices. The second and third control terminals follow the first and fourth control terminals, respectively. When the first device pushes, the third device pulls. When the first device pulls, the third device pushes. When the fourth device pushes, the second device pulls. When the fourth device pulls, the second device pushes. The second and third devices and the first and fourth devices are matched.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
FIG. 1 is an electrical schematic of a prior art emitter follower circuit;
FIG. 2 is an electrical schematic of a prior art common-emitter differential amplifier circuit;
FIG. 3 is an electrical schematic of a prior art class B output stage that operates in a push-pull fashion;
FIG. 4 is an electrical schematic of a buffer/amplifier circuit according to one embodiment of the present invention;
FIG. 5 is a waveform diagram illustrating a time lag between the output signals of the circuit of FIG. 4 when driving a capacitive load;
FIG. 6 is an electrical schematic of a buffer/amplifier circuit according to another embodiment of the present invention employing a degeneration capacitor;
FIG. 7 is a waveform diagram illustrating the effect of a degeneration capacitor on the output signals of the circuit of FIG. 6 when driving a capacitive load; and
FIG. 8 is an electrical schematic of a buffer/amplifier circuit according to another embodiment of the present invention.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The following description of preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. The same reference numerals are used to indicate similar elements or features throughout the drawings.
A buffer circuit according to one preferred embodiment of the present invention is illustrated in FIG. 4 and indicated generally by reference numeral 400. As shown in FIG. 4, the buffer circuit 400 includes four bipolar junction transistors Q10, Q11, Q12, and Q13. A positive input terminal V+ is coupled to the base of transistor Q11 as well as to the base of transistor Q12. A negative input terminal V− is coupled to the base of transistor Q10 as well as to the base of transistor Q13. A positive output terminal Vout+ is coupled to the emitter of transistor Q11 and to the collector of transistor Q13. A negative output terminal Vout− is coupled to the emitter of transistor Q10 and to the collector of transistor Q12. The collectors of transistors Q10, Q11 are coupled to supply voltage terminals Vcc, and the emitters of transistors Q12, Q13 are both coupled to one side of a bias current source 402. The other side of the current source 402 is coupled to ground.
As shown in FIG. 4, the positive input terminal V+ is preferably coupled to the bases of transistors Q11, Q12 through speed-up capacitors C11, C12, respectively. Similarly, the negative input terminal V− is preferably coupled to the bases of transistors Q10, Q13 through speed-up capacitors C10, C13, respectively.
In operation, transistors Q10 and Q11 essentially act as emitter followers, while transistors Q12 and Q13 function as a switch pair transconductance. The combined action of these two sets of transistors enhances the buffering action that each provides.
Assuming, for example, a sinusoidal input signal is applied across the positive and negative input terminals V+, V− and a load is connected across the positive and negative output terminals Vout+, Vout−, the circuit 400 of FIG. 4 generally operates as follows. When the positive input terminal V+ is pulled high, transistors Q11 and Q12 are biased on, while transistors Q10 and Q13 are biased off. Therefore, transistor Q11 pushes current to the load via the positive output terminal Vout+ while, at the same time, transistor Q12 pulls current from the load via negative output terminal Vout−. Conversely, when the negative input terminal V− is pulled high, transistors Q10 and Q13 are biased on, while transistors Q11 and Q12 are biased off. Therefore, transistor Q10 pushes current to the load via the negative output terminal Vout− while, at the same time, transistor Q13 pulls current from the load via the positive output terminal Vout+. Thus, the circuit of FIG. 4 simultaneously pushes current to one side of a load while pulling current from the other side of the load. In this manner, the load can be readily charged (or discharged), including when the load is capacitive and/or driven at high frequency.
By switching off transistor Q13 while transistor Q11 pushes current to the load via the positive output terminal Vout+, transistor Q11 is isolated from a path of the bias current, which generally flows at such time from the negative output terminal Vout− to ground through the transistor Q12 and the bias current source 402. Similarly, by switching off transistor Q12 while transistor Q10 pushes current to the load via the negative output terminal Vout−, transistor Q10 is isolated from the path of the bias current, which generally flows at such time from the positive output terminal Vout+ to ground through the transistor Q13 and the bias current source 402. Thus, unlike many prior art buffers, the transistor supplying current to one of the output terminals at a given time is not also required to supply the bias current. Accordingly, less current consumed.
It should also be noted that, during operation of the circuit of FIG. 4, there is a small period of time when transistors Q11 and Q13 are both conducting (and essentially interfering with one another), and a small period of time when transistors Q10 and Q12 are both conducting (and essentially interfering with one another). This is because the lower transistors Q12 and Q13 are generally biased on as their input signals climb above the reference voltage (i.e., the zero crossing), whereas the upper transistors Q10 and Q11 are generally biased on as their input signals rise above their peak negative values (i.e., before the zero crossing). As a result, the voltages at the positive and negative output terminals Vout+, Vout− are not exactly 180 degrees out of phase, as shown in FIG. 5. In other words, a positive voltage peak at one of the output terminals does not fully coincide with, but rather slightly lags in time, a negative voltage peak at the other output terminal. The positions of the peaks are a function of the capacitive load. This effect does not occur when the circuit is resistively loaded instead of capacitively loaded.
To compensate for the output phase difference illustrated in FIG. 5, a degeneration capacitor C14 can be coupled between the emitters of transistors Q12 and Q13, as shown in FIG. 6. Additionally, in the embodiment of FIG. 6, two bias current sources 602, 604 are provided in place of the single current source 402 of FIG. 4. As shown in FIG. 6, one of the current sources 602 is coupled to the emitter of transistor Q12, and the other current source 604 is coupled to the emitter of transistor Q13. By including the degeneration capacitor C14 in the circuit 600 of FIG. 6, transistors Q12 and Q13 are capacitively degenerated to produce sufficient phase shift such that transistors Q11 and Q13 are not biased on at the same time, and transistors Q10 and Q12 are not biased on at the same time. As a result, the voltages at the output terminals Vout+, Vout− are substantially 180 degrees out of phase, as illustrated in FIG. 7.
As an alternative to the circuit 600 of FIG. 6, which employs two current sources 602 and 604, the current source 602 can be replaced with a suitable resistor R3, as shown in FIG. 8. If the values of resistor R3 and capacitor C14 are relatively large, the impedance of the capacitor C14 will dominate at high frequency and the circuit 800 will function much like the circuit 600 of FIG. 6.
Those skilled in the art will appreciate that the buffer circuits of FIGS. 4, 6 and 8 can be readily configured as amplifiers, rather than unity gain buffers, if desired for any given application of the invention.
Preferably, transistors Q10 and Q11, as well as transistors Q12 and Q13, are matched transistor pairs having substantially the same operating characteristics. Even more preferably, all four transistors Q10–Q13 are matched and have the same performance characteristics.
For the specific embodiments shown in FIGS. 4, 6 and 8, transistors Q10–Q13 are NPN bipolar junction transistors (BJTs). It should be understood, however, that other types of transistors may be employed without departing the scope of the present invention.
One preferred application for the buffer/amplifier circuits of FIGS. 4, 6 and 8 is in video buffering, including buffering of video clock signals. The buffer/amplifier circuits may also be used to drive off-chip capacitive loads. Those skilled in the art will appreciate, however, that the teachings of the present invention are not so limited, and can be applied in a wide variety of applications.
Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.

## Claims

1. A differential circuit comprising:
first, second, third and fourth devices that have first, second, third and fourth control terminals, respectively wherein said second and fourth devices are arranged in series with said first and third devices, respectively;
a first output that communicates with said first device and said second device;
a second output that communicates with said third device and said fourth device;
a first input that communicates with said first control terminal;
a second input that communicates with said fourth control terminal,
wherein said first, second, third and fourth devices are the same type of devices;
wherein said third control terminal follows said first control terminal, and said second control terminal follows said fourth control terminal;
wherein when said first device pushes, said third device pulls;
wherein when said first device pulls, said third device pushes;
wherein when said fourth device pushes, said second device pulls;
wherein when said fourth device pulls, said second device pushes;
wherein said second and third devices are matched; and
wherein said first and fourth devices are matched.

2. The differential circuit of claim 1 wherein the first, second, third and fourth devices are matched.

3. The differential circuit of claim 1 wherein said first, second, third, and fourth devices comprise transistors.

4. The differential circuit of claim 1 further comprising first and second capacitances, wherein the first input communicates with the first and third control terminals through the first and second capacitances, respectively.

5. The differential circuit of claim 1 further comprising third and fourth capacitances, wherein the second input communicates with the second and fourth control terminals through the third and fourth capacitances, respectively.

6. The differential circuit of claim 1 further comprising a capacitance that communicates with the first and fourth devices.

7. The differential circuit of claim 6 further comprising:
a first bias current source that communicates with the first device and one end of the capacitance; and
a second bias current source that communicates with the fourth device and an opposite end of the capacitance.

8. The differential circuit of claim 1 further comprising at least one bias current source that communicates with the first and fourth devices.

9. The differential circuit of claim 3 wherein the transistors comprise bipolar junction transistors.

10. The differential circuit of claim 1 wherein the differential circuit implements one of a unity gain buffer and an amplifier.