Patent Publication Number: US-7224010-B2

Title: Voltage-controlled amplifier for a signal processing system

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention provides a voltage-controlled amplifier, and more particularly, a voltage-controlled amplifier with low noise, low distortion, high integration degree, and low cost. 
   2. Description of the Prior Art 
   A voltage-controlled amplifier, or VCA, can be seen as a three-terminal unit, including an input end, an output end, and a control signal reception end, utilized for changing a gain according to a control signal. The VCA is usually used in a multimedia electric device, such as a DVD player, a VCD player, a portable video player, a digital TV, etc. A user can adjust volume, contrast, brightness, or a channel of the multimedia electric device through the VCA. 
   The VCA has become more and more important owing to a variety of signal types. The prior art VCAs (voltage-controlled amplifiers) are manufactured with a bipolar process or a BiCMOS (bipolar complementary metal oxide semiconductor) process. A VCA, manufactured with the bipolar process, can operate in a high speed and drive a high current, but cannot be integrated with a digital circuit. In comparison, a VCA, manufactured with the BiCMOS process, combines advantages of a BJT and a CMOS, including high speed, high current driving, low power, high input impedance, high noise margin, etc. However, cost of the BiCMOS manufacturing process is too high. Therefore, a VCA with low noise, low distortion, high integration degree, and low cost is needed. 
   SUMMARY OF THE INVENTION 
   It is therefore a primary objective of the claimed invention to provide a voltage-controlled amplifier for a signal processing system. 
   According to the claimed invention, a voltage-controlled amplifier for a signal processing system comprises an input voltage reception end, a first voltage-to-current converter, a reference current generator, a gain adjustment circuit, a first current mirror, and an output circuit. The input voltage reception end is utilized for receiving an input voltage. The first voltage-to-current converter is coupled to the input voltage reception end, and is utilized for outputting a first current according to the input voltage received by the input voltage reception end. The reference current generator is utilized for generating a second current. The gain adjustment circuit is coupled to the first voltage-to-current converter and the reference current generator, and is utilized for receiving the first current and the second current, and adjusting a gain of the voltage-controlled amplifier. The gain adjustment circuit comprises a first bipolar junction transistor, a second bipolar junction transistor, a third bipolar junction transistor, a fourth bipolar junction transistor, and a control voltage reception circuit. Each of the bipolar junction transistors comprises a collector, a base end, and an emitter. The control voltage reception circuit comprises an end coupled to the base of the second bipolar junction transistor and the base of the third bipolar junction transistor, and the other end coupled to the base of the first bipolar junction transistor and the base of the fourth bipolar junction transistor, for outputting a control voltage. The first current mirror is coupled to the first voltage-to-current converter, the gain adjustment circuit, and the reference current generator, and comprises a reference branch, a drain branch, and a mirror branch. The reference branch is coupled to an output end of the first voltage-to-current converter, and is utilized for transmitting the first current. The drain branch is coupled to the reference current generator, the emitter of the first bipolar junction transistor and the emitter of the second bipolar junction transistor, and is utilized for draining a current equal to the first current from the emitter of the first bipolar junction transistor and the emitter of the second bipolar junction transistor. The mirror branch is coupled to the emitter of the third bipolar junction transistor and the emitter of the fourth bipolar junction transistor, for providing a current equal to the first current for the emitter of the third bipolar junction transistor and the emitter of the fourth bipolar junction transistor. The output circuit is coupled to the gain adjustment circuit, and is utilized for determining a difference value between a current of the collector of the first bipolar junction transistor and a current of the collector of the fourth bipolar junction transistor, for outputting an output voltage. The output voltage outputted from the output circuit is controlled according to the control voltage outputted from the gain adjustment circuit. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a schematic diagram of a VCA in accordance with a preferred embodiment of the present invention. 
       FIG. 2  illustrates a schematic diagram of a prior art parasitic lateral BJT. 
       FIG. 3  illustrates a schematic diagram of an exponential amplifier in accordance with the present invention. 
       FIG. 4  illustrates a schematic diagram of a stable-amplitude oscillator in accordance with the present invention. 
       FIG. 5  illustrates an adjustable low-pass filter in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 1 , which illustrates a schematic diagram of a VCA  100  in accordance with a preferred embodiment of the present invention. The VCA  100  includes an input voltage reception end, a first voltage-to-current converter  102 , a reference current generator  160 , a gain adjustment circuit  106 , a first current mirror  112 , and an output circuit  108 . The VCA  100  can adjust a gain according to a variable control voltage V C , and amplify an input voltage V in  received from the input voltage reception end to an output voltage V out  with the adjusted gain. When adjusting the gain, the present invention changes an alternating-current (AC) portion of the input voltage V in , but does not change a direct-current (DC) portion of the input voltage V in . Also, the present invention can decrease noise, distortion, and increase integration degree. The detailed operation of the VCA  100  will be described in the following. 
   The first voltage-to-current converter  102  includes a first operational amplifier OP 1 , a NMOSFET (n-type metal oxide semiconductor field effect transistor)  124 , and a first resistor R 1 , utilized for generating a first current according to the input voltage V in . The reference current generator  160  includes a second voltage-to-current converter  104 , a reference voltage generator  150 , and a fourth current mirror  110 . The second voltage-to-current converter  104  includes a second MOS (metal oxide semiconductor) transistor  126 , a second operational amplifier OP 2 , and a second resistor R 2 , utilized for transforming a reference voltage V ref  generated by the reference voltage generator  150  into a second current. The second MOS  126  includes a gate, a source, and a drain. The drain of the second MOS  126  is coupled to a branch of the fourth current mirror  110 . The second operational amplifier OP 2  includes a first input end coupled to the reference voltage generator  150 , a second input end coupled to the source of the second MOS  126 , and an output end coupled to the gate of the second MOS  126 . The second resistor R 2  is coupled between the source of the second MOS  126  and the second input end of the second operational amplifier OP 2 . In an embodiment, the second MOS  126  is a PMOSFET (p-type metal oxide semiconductor field effect transistor). The reference voltage generator  150  preferably includes a voltage source and a series of two resistors R coupled to the ground, utilized for generating the reference voltage V ref . The fourth current mirror  110  is coupled to the second voltage-to-current converter  104  and the gain adjustment circuit  106 , utilized for receiving and transmitting the second current generated by the second voltage-to-current converter  104  to the gain adjustment circuit  106 . 
   The first and second operational amplifiers OP 1  and OP 2  preferably are manufactured with a CMOS process, so the first and second operational amplifiers OP 1  and OP 2  have advantages of high input impedance and low thermal noise, which ensures correct operation when the input voltage V in  is extremely high, and decreases noise and distortion. The first voltage-to-current converter  102  transforms the input voltage V in  into the first current, and the second voltage-to-current converter  104  transforms the reference voltage V ref  into the second current. The resistance of the first resistor R 1  in the first voltage-to-current converter  102  is twice the resistance of the second resistor R 2  in the second voltage-to-current converter  104 , so that when a DC level of the input voltage V in  equals a DC level of the reference voltage V ref , the second current outputted from the second voltage-to-current converter  104  is twice the first current outputted from the first voltage-to-current converter  102 . If the input voltage V in  includes an AC signal, then the output current of the first voltage-to-current converter  102  includes a difference Δi. In other words, the second current outputted from the second voltage-to-current converter  104  is  21 , and the first current outputted from the first voltage-to-current converter  102  is (I+Δi). 
   The gain adjustment circuit  106  includes first, second, third, and fourth BJTs (bipolar junction transistors) Q 1 , Q 2 , Q 3 , and Q 4 , and a control voltage reception circuit  122 . In order to decrease the production cost, preferably, the first, second, third, and fourth BJTs Q 1 , Q 2 , Q 3 , and Q 4  are parasitic lateral BJTs manufactured in a CMOS process. Please refer to  FIG. 2 , which illustrates a schematic diagram of a prior art parasitic lateral BJT  200 . As those skilled in the art recognize, the parasitic lateral BJT  200  includes a p-type extrinsic base, an n-type collector and a p-type extrinsic base of an n-p-n junction. The word “lateral” means that current laterally flows from an emitter to the collector in a silicon crystal surface, resulting in a larger base impedance, fast reaction speed, and facility for combining with other CMOS circuits. Go back to  FIG. 1 , the second voltage-to-current converter  104  outputs the current  21  to emitters of the first and second BJTs Q 1  and Q 2 . The first voltage-to-current converter  102  outputs the current (I+Δi) mirrored by the first current mirror  112  to emitters of the third and fourth BJTs Q 3  and Q 4 , and drains the current (I+Δi) from the emitters of the first and second BJTs Q 1  and Q 2 . Therefore, current flowing into the emitters of the first and second BJTs Q 1  and Q 2  is (I−Δi), and current flowing into the emitters of the third and fourth BJTs Q 3  and Q 4  is (I+Δi). Suppose that collector currents of the first, second, third, and fourth BJTs Q 1 , Q 2 , Q 3 , and Q 4  are I C1 , I C2 , I C3 , and I C4 , and voltages from bases to emitters of the first, second, third, and fourth BJTs Q 1 , Q 2 , Q 3 , and Q 4  are V BE1 , V BE2 , V BE3 , V BE4 , then:
 
−V C =V BE1 −V BE2 =V T ln(I C1 /I C2 )
 
−V C =V BE3 −V BE4 =V T ln(I C1 /I C2 )
 
and
 
I C2 =I−Δi−I C1 
 
I C3 =I+Δi−I C4 
 
so
 
−V C =V T ln(I C1 /(I−Δi−I C1 ))
 
−V C =V T ln(I C4 /(I+Δi−I C4 ))
 
then
 
I C1 =(I−Δi)/(1+exp(V C /V T ))
 
I C4 =(I+Δi)/(1+exp(V C /V T ))
 
wherein V T  is thermal voltage
 
   In addition, the output circuit  108  outputs an output current I O  according to the collector currents I C1  and I C4 . The output circuit  108  preferably includes a third current mirror  116 , a second current mirror  120 , and an output resistor R O . The third current mirror  116  and the second current mirror  120  mirrors the currents I C1  and I C4  to the output resistor R O  respectively. So,
 
I O =I C4 −I C1 =(2Δi)/(1+exp(V C /V T ))
 
then a gain of the VCA  100  is
 
ΔV out /ΔV in =(I O ×R O )/(Δi×R1)=(R O /R1)×(2/(1+exp(V c /V T ))
 
   Therefore, the gain of the VCA  100  changes in response to the control voltage V C . That is, by adjusting the control voltage V C  of the gain adjustment circuit  106 , the VCA  100  adjusts the output voltage of the output circuit  108 . 
   Moreover, in the VCA  100 , adjusting the control voltage V C  only changes the AC part of the output signal V out . Since the first, second, third, and fourth BJTs Q 1 , Q 2 , Q 3 , and Q 4  are parasitic lateral BJTs manufactured in the CMOS process, the production cost can be reduced, the base impedances are high, the reaction speed is fast, and it is easy to integrate with the other CMOS circuits. Furthermore, the high input impedance and low thermal noise of the first and second operational amplifiers OP 1  and OP 2  can bear the high input voltage V in  and decrease noise and distortion. Therefore, the VCA  100  is suitable for handling large signals. 
   Therefore, those skilled in the art can use the present invention VCA  100  to implement application circuits for decreasing noise, distortion, and cost, and increasing integration degree. For example, please refer to  FIG. 3 ,  FIG. 4 , and  FIG. 5 .  FIG. 3  illustrates a schematic diagram of an exponential amplifier  300 ,  FIG. 4  illustrates a schematic diagram of a stable-amplitude oscillator  400 , and  FIG. 5  illustrates an adjustable low-pass filter  500 . In  FIG. 3 , the exponential amplifier  300  receives a reference voltage V in  from the input end of the VCA  100 , and amplifies an input voltage V in ′ to an exponential time. In  FIG. 4 , the stable-amplitude oscillator  400  changes the control voltage V C  of the VCA  100  by changing voltages V R  and V−, and outputs an oscillating signal with an amplitude V R  through the output end of the VCA  100 . In  FIG. 5 , by adjusting the control voltage V C  of the VCA  100 , the adjustable low-pass filter  500  changes a pass-band width. The circuits in  FIG. 3 to 5  are applications of the present invention, and do not limit the present invention. 
   In summary, the gain of the present invention VCA is changed in response to the control voltage and only amplifies the AC portion of the output signal. Moreover, the BJTs in the gain adjustment circuit are parasitic lateral BJTs manufactured in the CMOS process, so as to decrease noise, distortion, and cost, and increase integration degree. Furthermore, the high input impedance and low thermal noise of the operational amplifiers ensures correct operation when the input voltage is too high, and decreases noise and distortion. Therefore, the present invention VCA combines advantages of low noise, low distortion, low cost, and high integration degree. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.