Bias circuit for a low voltage differential circuit

[0002] The present invention relates to differential circuits and techniques, and more particularly to a bias circuit for low voltage differential applications.

[0003] The bipolar junction transistor (BJT) differential pair is one of the most important integrated circuit (IC) building blocks. The emitters of two NPN BJ transistors Q1, Q2 are connected together and to a constant current sink to ground. The supply voltage is connected to the collectors of the transistors through separate bias resistors, which may have matching resistances. A differential input signal (V_IN+, V_IN−) is applied across the bases of the differential pair, and a differential output voltage (V_OUT+, V_OUT−) is developed across the collectors of the differential pair. The current sink establishes a bias current I_BIASthat sets the total of the sum of emitter currents for transistor differential pair Q1 and Q2. Usually, the base currents are relatively small (i.e., high current gain) so the sum of the collector currents (I_C1and I_C2) of the transistors Q1 and Q2 is also equal to the current from the current sink. This results in the well-known hyperbolic tangent function “tanh” for the differential collector currents as illustrated by the following equations 1 and 2:

[0004] (I_C1+I_C2)=I_BIAS  (EQ 1)

[0005] (I_C1−I_C2)=(I_BIAS) tanh [V_D/2V_T], V_D=(V_IN+−V_IN−)  (EQ 2)

[0006] where V_Tis a thermal coefficient voltage (the voltage equivalent of temperature), where V_T=kT/q, where “k” is the Boltzmann constant in joules per degree Kelvin, T is the temperature in degrees Kelvin (absolute scale), and “q” is the magnitude of the charge of an electron.

[0007] The current sink bias sets the value of the sum of the collector currents of the transistors. The differential collector current depends upon the differential base voltages, but its maximum value is also limited by the current sink bias. A DC supply of base current is usually required for this circuit to work. Sometimes the DC base current is supplied by the signal source itself, but if the signal is AC coupled to the base, as is often the case for radio frequency (RF) circuits, for example, then a separate DC bias path is used to supply DC base current. In this case, the DC base bias circuit usually has an AC impedance value that is large compared to the impedance of the base terminals of the BJTs, so that little signal is lost to this base bias circuit.

[0008] For circuits operating with supply voltages less than 3 volts (V), the differential pair uses a large percentage of the total available supply voltage. The current sink itself may consume nearly a volt.

[0009] A bias circuit according to embodiments of the present invention provides biasing for a differential circuit, where the differential circuit includes a pair of differential transistors having control inputs receiving a differential signal and current paths coupled between a source signal and ground. The bias circuit includes first and second transistors, first and second impedance devices, a reference current source and an amplifier. The first and second transistors each have a control input and further have a current path coupled between a first node and ground. The control inputs of the first and second transistors receive the differential signal. The first and second impedance devices are each coupled between a control input of one of the first and second transistors and a second node. The reference current source provides a reference current for the first node and the amplifier has an input coupled to the first node and an output coupled to the second node. In this manner, the differential circuit need not include a constant current source in series with the differential transistors so that the differential circuit may be utilized in low voltage applications.

[0010] An inductor may be coupled in the current path of the differential transistors between the first and second transistors and ground. The impedance of an inductor is low at low frequencies and thus has little or no effect. The impedance of the inductor is, however, high at high frequencies. In this manner, the inductor operates in a similar manner as a constant current source at high frequencies so that the circuit responds in a similar manner as a standard differential circuit. A filter capacitor may be coupled between the first node and ground and operates as a low pass filter.

[0011] In more specific configurations, the first and second transistors each comprise a bipolar junction transistor (BJT) having a base input and further having a current path formed by a collector and an emitter. Also, the first and second transistors may each comprise NPN transistors. Also, the emitters of the differential transistors of the differential circuit and the first and second transistors may each be connected to ground. The differential transistors of the differential circuit may also be BJTs, where the first and second transistors have emitter area ratios that are matched to the emitter area ratios of the differential transistors of the differential circuit. It is noted that PNP or metal oxide semiconductor transistor (MOS) configurations are also contemplated.

[0012] The first and second impedance devices may be resistors or inductors. The reference current may be fixed, or may be modulated to make the circuit into a type of mixer or multiplier. Also, the impedance devices may be replaced by current sources or mixers, which are controlled by either of the first or second nodes. Further, additional pairs of transistors may be added to obtain additional pairs of output currents. Emitter generation may be added between each transistor emitter and ground. The amplifier may be a non-inverting amplifier, a buffer, an emitter-follower, or even a short circuit in some configurations.

[0013] A biased differential circuit according to the present invention includes first and second pairs of differential transistors, first and second resistors, first and second impedance devices, a reference current source and an amplifier. The first pair of differential transistors each has a control input and first and second current terminals, where the first terminal of each is coupled to ground and where the control inputs of the pair of differential transistors receive the differential input signal. The first and second resistors are each coupled between the source signal and the second current terminal of a respective one of the first pair of differential transistors. The second current terminals of the first pair of differential transistors develops a differential output signal. The second pair of differential transistors each have a control input and first and second current terminals coupled between a first node and ground. The control inputs of the second pair of differential transistors receive the differential input signal. The first and second impedance devices are each coupled between a control input of one of the second pair of differential transistors and a second node. The reference current source is coupled to the source signal and provides a reference current for the first node. The amplifier has an input coupled to the first node and an output coupled to the second node. The variations previously described are applicable in a similar manner.

[0014] A better understanding of the present invention can be obtained when the following detailed description of embodiments of the invention is considered in conjunction with the following drawings, in which:

[0015] FIG. 1 is schematic and block diagram of a bias circuit for biasing a low voltage differential circuit.

[0016] FIG. 2 is a schematic diagram of an exemplary embodiment of the bias circuit of FIG. 1 coupled to the differential circuit.

[0017] FIG. 3 is a schematic diagram of another exemplary embodiment of the bias circuit, which includes an inductor connected between the emitters of the emitter-coupled transistors and ground.

[0018] FIG. 1 is schematic and block diagram of a bias circuit 101 for biasing a low voltage differential circuit 103. The differential circuit 103 is similar to a typical differential circuit except that the constant current bias sink is removed. In particular, the low voltage differential circuit 103 includes a differential pair of NPN bipolar junction transistors (BJT) Q1 and Q2 having their emitters connected together and to ground (Gnd). The supply voltage V_Supplyis connected to the collectors of the transistors through separate bias resistors R1 and R2, which may have matching resistances. A differential input signal (V_IN+, V_IN−) is applied across the bases and a differential output voltage (V_OUT+, V_OUT−) is developed across the collectors of the differential pair of transistors Q1 and Q2. The collector currents of the transistors Q1 and Q2 are I_C1and I_C2, respectively.

[0019] It is noted that other types of transistors may be employed, such as metal oxide semiconductor (MOS) transistors, field effect transistors (FETs), PNP transistors, etc. The base of the NPN BJTs is considered an input or control terminal whereas the collectors and emitters form a controlled current path for each transistor. A similar analysis applies for MOS transistors or FETs, where the gate is an input or control terminal whereas the source and drain form a controlled current path.

[0020] The bias circuit 101 includes four terminals for coupling to the differential circuit 103. In particular, the bias circuit 101 includes “Vsup” and “Gnd” terminals for coupling to the V_Supplyand ground signals, respectively, for supplying voltage and reference. The bias circuit 101 also includes “Vpos” and “Vneg” terminals for coupling to the V_IN+ and V_IN− signals, respectively, of the differential circuit 103. As described more fully below, the bias circuit 101 effectively replaces the constant current source typically coupled between the transistor emitters and ground and performs a similar function. The bias circuit 101, however, provides the benefit of eliminating the voltage drop that would otherwise exist across the current source, thereby providing a greater amount of voltage swing for the differential circuit 103. This is particularly beneficial for low voltage applications, such as when the V_Supplysignal is on the order of 3 volts (V) or less. The bias circuit 101 also eliminates base current error that otherwise exists in typical emitter-coupled, differential pair circuits since the collector currents in the typical circuits are smaller than the emitter currents by the amount of the base current.

[0021] FIG. 2 is a schematic diagram of an exemplary embodiment of the bias circuit 101 coupled to the differential circuit 103. The differential circuit 103 is essentially identical with the same elements Q1, Q2, R1 and R3 coupled between the V_Supplyand ground signals in the same manner. In the embodiment shown, the bias circuit 101 includes two BJTs Q3 and Q4, two resistors R3 and R4, a constant current source 201 asserting a reference current I_REF, a non-inverting amplifier 203 and an optional filter capacitor C1. In the particular configuration shown, the transistors Q3 and Q4 are coupled to the transistors Q1 and Q2 with their emitters coupled to ground. The base of the transistor Q3 is coupled to the base of the transistor Q1 for receiving the V_IN+ signal. The base of the transistor Q4 is coupled to the base of the transistor Q2 for receiving the V_IN− signal. The collectors of the transistors Q3 and Q4 are both coupled to a voltage node V₁(with voltage V₁). The resistor R3 is coupled between the base of the transistor Q3 and another voltage node V₂(with voltage V₂), and the resistor R4 is coupled between the base of the transistor Q4 and the V₂node. The current source 201 is coupled between the V_Supplysignal and the V₁node for injecting the I_REFcurrent into the V₁node. The amplifier 203 has its input coupled to the V₁node and its output coupled to the V₂node. The optional capacitor C1 is coupled between the V₁node and ground.

[0022] In the embodiment shown, the transistors Q1-Q4 are substantially similar or otherwise identical. It is desired that the DC impedance (resistance) of the sources (driving V_IN+ and V_IN−) be finite and preferably large. For example, it is sufficient if the sources are AC coupled, such as connected through capacitors or the like, to the bases of the transistors Q1 and Q2. In addition, it is generally desirable that the impedance of the bias circuit 101, including the resistors R3 and R4, be large compared to the source and/or base impedance of the transistors Q3 and Q4 at signal frequencies. Exemplary signal frequencies are in the gigahertz (GHz) frequency range.

[0023] In operation, the sum of the collector currents I_C3and I_C4of the transistors Q3 and Q4 is compared to the desired reference current I_REF. The difference between the collector current sum (I_C3+I_C4) and the reference current I_REFis converted to an error voltage or current by the amplifier 203, where the error is applied back to the bases of the transistors Q3 and Q4 through isolating impedances established by the resistors R3 and R4. Assuming the transistors Q1-Q4 are similar, then the collector currents I_C1and I_C3are approximately equal, and the collector currents I_C2and I_C4are also approximately equal. The reference current I_REFflows into the V₁node and the collector currents I_C3and I_C4flow out of this same node V₁. If the reference current I_REFis larger than the sum of the collector currents I_C3and I_C4, then the node voltage V₁becomes more positive, which causes the node voltage V₂to also become more positive through operation of the amplifier 203. At any moment in time, the input voltages V_IN+ and V_IN− may not be equal. As the node voltage V₂becomes more positive, however, the current through R3 and R4 increases so that the average input voltage becomes more positive. As the average input voltage becomes more positive, the sum of the collector currents I_C3and I_C4becomes greater. The circuit stabilizes when the sum of the collector currents I_C3and I_C4equals the reference current I_REF. This feedback loop continuously maintains the average input voltage even in the presence of separately varying inputs.

[0024] The optional capacitor C1 returns net alternating current (AC) from the collector currents I_C3and I_C4to ground thereby acting as a low pass filter. The amplifier 203 may be replaced by a buffer, such as an emitter-follower or the like, or, in some instances, may be replaced by a short-circuit so that the nodes V₁and V₂are the same.

[0025] For low frequency operation, such as when the variation of the input voltage V_IN+ and V_IN− is slow relative to the bias feedback loop, then at each instant of time the following equations 3-8 illustrate operation:

[0026] I_C3+I_C4=I_REF, I_C1=I_C3, I_C2=I_C4  (EQ 3)

[0027] I_C1/I_C2=exp[V_D/V_T], V_D=(V_IN+−V_IN−)  (EQ 4)

[0028] I_C1/(I_REF−I_C1)=exp[V_D/V_T], I_C2/(I_REF−I_C2)=exp[−V_D/V_T]  (EQ 5)

[0029] I_C1=(I_REF−I_C1)exp[V_D/V_T], I_C2=(I_REF−I_C2)exp[−V_D/V_T]  (EQ 6)

[0030] I_C1(1+exp[V_D/V_T])=(I_REF)exp[−V_D/V_T],

[0031] I_C2(1+exp[−V_D/V_T])=(I_REF)exp[−V_D/V_T]  (EQ 7)

[0032] I_C1={(I_REF)exp[V_D/V_T]}/{1+exp[V_D/V_T]},

[0033] I_C2={(I_REF)exp[−V_D/V_T]}/{1+exp[−V_D/V_T]}  (EQ 8)

[0034] where “exp” is the exponential function (natural logarithm). It is also noted, according to the following equation 9, that:

[0035] (exp[V_D/V_T])(exp[−V_D/2V_T])=exp[−V_D/2V_T]  (EQ 9)

[0036] So, according to the following equation 10, it is true that:

[0037] I_C1={(I_REF)exp[V_D/2V_T]}/{exp[−V_D/ 2V_T]+exp[V_D/2V_T]},

[0038] I_C2={(I_REF)exp[−V_D/2V_T]}/{exp[V_D/ 2V_T]+exp[−V_D/2V_T]}  (EQ 10)

[0039] Calculating the differential current results in the following equation 11: $\begin{array}{cc}\begin{array}{c}{I}_{\mathrm{C1}}-I_{\mathrm{C2}}=\left(I_{\mathrm{REF}}\right)\left\{\exp \left[V_D/2V_T\right]-\exp \left[-V_D/2V_T\right]\right\}/\\ \left\{\exp \left[V_D/2V_T\right]+\exp \left[-V_D/2V_T\right]\right\}\\ =\left(I_{\mathrm{REF}}\right)\tanh \left(V_D/2V_T\right)\end{array}& \left(\mathrm{EQ}11\right)\end{array}$

[0040] This is a similar function as the common differential pair biased with a current sink, except that there is no base current error for the bias circuit 101.

[0041] The bias circuit 101 interacts with the input signals V_IN+ and V_IN− to transform the effective input into a differential signal. For example, suppose a positive going signal was applied to the positive input V_IN+ but no signal was applied to the negative input V_IN−. The bias circuit 101 adjusts the voltage on both bases in the negative direction so that the base voltage at Q2 goes negative by the amount of half the value of the applied signal and that the base voltage at Q1 goes positive by only half the applied input.

[0042] For high frequency operation, such as when the input changes faster than the feedback circuit can follow, then the bias circuit 101 acts to maintain the average (or DC) of the sum of the collector currents equal to the reference current I_REF. For low amplitude inputs (such as below the thermal coefficient voltage V_T), the transistors Q1 and Q2 respond independently to their respective inputs, i.e., the collector current I_C1depends only upon the input voltage V_IN+ and the collector current I_C2depends only upon the input voltage V_IN− with the DC bias equal to half of the reference current I_REF. In many applications, the input voltages V_IN+ and V_IN− are differential in nature so the collector currents are also differential.

[0043] A large amplitude input (such as greater than the thermal coefficient voltage V_T) applied to the base of one of the transistors Q1, Q2 increases the DC bias of that transistor. The bias circuit 101 senses the increase in DC and reduces the DC voltage at the bases of all the transistors Q1-Q4. In this manner, the input voltage on one side affects the collector currents on the other side. Since the bias circuit 101 fixes the average current, it is useful for converting large amplitude sine waves into square waves.

[0044] FIG. 3 is a schematic diagram of a bias circuit 300, which is similar to the bias circuit 101 with the addition of an inductor L1 connected between the emitters of the transistors Q1, Q2, Q3, Q4 and ground. The impedance of the inductor L1 is low at low frequencies and high at high frequencies. At low frequencies, the bias circuit 300 operates as described previously since the inductor L1 has no effect. At high frequencies, however, the impedance of the inductor L1 acts like a constant current source and the bias circuit 300 responds in a similar manner as a traditional emitter-coupled, differential circuit configuration.

[0045] Many other variations and embodiments of the bias circuits 101 and 300 are possible and useful and are within the scope of the present invention. In one variation, the transistor emitter area ratios (Q1/Q3) are modified to other than 1:1 to scale the Q1/Q2 bias current relative to the reference current I_REF. In another variation, additional pairs of transistors are added to obtain more than one pair of output currents. In another variation, emitter degeneration can be added between each emitter and ground. In another variation, the resistors R3 and R4 are replaced by inductors. In another variation, the resistors R3 and R4 are replaced by current sources or current mirrors which are controlled by the voltages V₁or V₂. In another variation, the NPN transistors are replaced by PNP transistors. In another variation, the BJTs are replaced by metal oxide semiconductor (MOS) transistors or the like. In another variation, the reference current I_REFis modulated for mixer or multiplier type operation. Of course, any one or more of these variations may be combined for achieving additional embodiments without going beyond the spirit and scope of the present invention.

[0046] It is appreciated that a bias circuit in accordance with one or more embodiments of the present invention is suitable for providing bias functions for a differential circuit. The standard constant current sink is eliminated thereby enabling a greater signal voltage swing, which is particularly advantageous for low voltage applications. The bias circuit generally operates in parallel with the differential circuit rather than the series configuration required by the current sink. The bias circuit also eliminates base current error, and has many other useful functions depending upon frequency of operation.

[0047] Although a system and method according to the present invention has been described in connection with one or more embodiments, it is not intended to be limited to the specific form set forth herein, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents, as can be reasonably included within the spirit and scope of the invention as defined by the appended claims.