WO2005039045A1

WO2005039045A1 - Electronic circuit for voltage to current amplification

Info

Publication number: WO2005039045A1
Application number: PCT/IB2004/051985
Authority: WO
Inventors: Ronaldus J. M. Van Boxtel; Cornelis A. A. Hagedoorn
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-10-15
Filing date: 2004-10-06
Publication date: 2005-04-28

Abstract

The present invention provides a RF amplifier having matching transistors. The sources of the transistors are coupled to a high impedance current source. This way second order harmonic distortions and, in case of precisely matching transistors, also higher even-order harmonic distortions are cancelled out.

Description

Electronic circuit for voltage to current amplification

The present invention relates generally to amplifiers and, more particularly to voltage to current amplifiers for high frequency signals.

High frequency amplifiers are used for various applications, such as for cable antenna television (CATV) systems. CATV systems require wide-band amplifiers that amplify signals across a broad frequency bandwidth. However, high frequency amplifier applications extend operation into areas where parasitic effects of inter-electrode capacitance, wire inductance, stored charge and even operating frequency wave length begins to adversely affect circuit behavior. Minimizing distortion is particularly important when a series of amplifiers is cascaded over a signal transmission path, such as a series of high frequency amplifiers in a CATV transmission network. In an ideal communication system it is preferable that the components that comprise the system are linear. However, as a practical reality, there are many non linearities that are typically introduced by the high frequency amplifiers. The distortions created by a high frequency (RF) amplifier which are of primary concern are second order and higher even-order harmonic distortions and third order harmonic distortion components. Prior art amplifier designs have attempted to ameliorate the effects of even- order distortions by employing push-pull amplifier topologies, since the maximum even- order cancellation occurs when the proper 180 degree phase relationship is maintained over the entire bandwidth. This is often achieved through equal gain in both push-pull (halves) by matching the operating characteristics of the active devices. Such a push-pull wide band amplifier is disclosed in US patent no. 6,01 1,438. This amplifier includes a divider which divides the input signal to be amplified into two signals of differing phase and two identical amplifying circuits for amplification of the two divided signals. After amplification a combiner combines the two signals of the amplifying circuits into one signal and outputs the result signal. It is a disadvantage of such prior art push-pull wide band amplifiers that a divider and a combiner as well as two identical amplifiers are required for cancellation of second-order distortion products. This substantially adds to the complexity and cost of such prior art amplifiers.

The present invention provides for an electronic circuit for voltage to current amplification of a high frequency signal. The electronic circuit comprises two transistors which are substantially identical and have equivalent electrical properties. Preferably field effect transistors are used as these are produced with lower tolerances in comparison to bipolar transistors. One of the transistors receives the high frequency signal to be amplified at its control terminal. The drain terminal of this transistor is coupled to virtual ground. Preferably this is accomplished by connecting the drain terminal to a supply voltage via a decoupling capacitor. The other transistor provides the amplified output signal at its drain terminal. The control terminal of this transistor is AC coupled to ground. The source terminals of both transistors are connected to a common point where a high impedance current source is applied. An inductance is coupled between the drain terminal which provides the supply voltage. By means of the inductance it is prevented that high frequency signal components of the amplified high frequency signal are added to the supply voltage which would otherwise negatively impact the linearity of the amplification. It is a particular advantage of the present invention that second-order and higher even-order harmonic distortions are cancelled out without a need to divide the input signal to be amplified, without a need for two separate amplifiers and without having to recombine the separately amplified signals. In comparison to the prior art the electronic circuit of the invention can thus be realized with a minimal number of components.

In the following preferred embodiments of the invention will be described in greater detail by making reference to the drawings in which: Fig. 1 is a circuit diagram of a preferred embodiment of an electronic circuit of the invention, Fig. 2 is an equivalent circuit diagram of the electronic circuit of figure 1 illustrating the cancellation of even-order harmonic distortions.

Figure 1 shows electronic circuit 100. Electronic circuit 100 has transistor Q\ and transistor Q . Transistor Qi has control terminal 102 for applying a RF signal V_ιnι to be amplified. Drain terminal 104 of transistor Qi is coupled to supply voltage Vcc and is AC decoupled by means of capacitor 106. This way drain terminal 104 is coupled to virtual ground. Transistor Q₂ provides the amplified output signal V_out at its drain terminal

108. Control terminal 110 of transistor Q₂ is AC coupled to ground. DC current source 112 is coupled to the source terminals of both transistors Qi and Q₂, i.e. source terminal 114 and source terminal 116. DC current source 112 has high impedance R_E. Drain terminal 108 of transistor Q₂ is coupled to the supply voltage Vcc by means of inductance 118. It is to be noted that transistors Qi and Q₂ are identical and have thus equivalent electrical and thermal properties. Preferably transistors Qi and Q₂ are field effect transistors as field effect transistors are more ideal voltage to current amplifiers, resulting to better DC matching of transistors Ql and Q2 in comparison to bipolar transistors. Besides the better equal DC settings for FET's, also the better voltage to current amplification in FET's yields in a substantially better common mode cancellation behavior. In operation input voltage V_ιnι is applied between control terminal 102 and ground. A peak to peak (p-p) voltage swing of input voltage V,„ι which is applied at control terminal 102 is illustrated in figure 1. DC current source 112 draws a total current I from the transistors Qi and Q₂.

As the transistors Qi and Q₂ are substantially identical the current I is split in half such that current 1/2 is drawn from source terminal 114 and current 1/2 is drawn from source terminal

1 16. As a consequence there is a voltage drop of V,„ι/2 between control terminal

102 and source terminal 114 of transistor Qi and a voltage drop having the same magnitude but opposite direction at transistor Q, i.e. voltage drop V,„ι/2 between source terminal 1 16 and control terminal 110. This results in identical second order harmonic distortions, i.e. error signals e, which are produced by the non linear behavior of transistor Ql and Q2 though are cancelled in the virtual common mode point, the source terminals 114 and 116 of transistors Qi and Q₂ respectively or the drain terminal of current source 112 in case source degeneration resistance's are used in the source terminals of transistor Ql and Q2 respectively . These second order harmonic distortions are cancelled out which provides the linear amplification of input signal V,_nι. The resulting peak to peak wave form of the resulting amplified output signal V_out is also illustrated in the drawing. When transistor Q] and Q₂ are exactly matching also higher even-order harmonic distortions are cancelled out. Another advantage is that the cancellation is accomplished even in case of temperature variations as the voltage drops between the control and source terminals of transistors Qi and Q₂ are of opposite direction. It is a further advantage that power dissipation is minimized in comparison to prior art push-pull high frequency amplifiers. It is to be noted that the output provided at drain terminal 108 is non-inverting, which is an advantage for most applications. However, if an inverting output is required this can be obtained at the drain terminal 104 of transistor Qi, though then the required inductance 1 18 will be placed between Vcc and drain 104 where drain terminal 108 will be connected to Vcc and will be AC decoupled by capacitor 106.. The equivalent circuit diagram of figure 2 shows equivalent transformer 120 which has primary winding 122 and secondary winding 124. Transistor Qi provides half the amplified output signal V_out /2 and second order harmonic distortion signal e whereas transistor Q₂ provides the inverted output signal V_out /2 and substantially the same second order harmonic distortion signal e. The signals provided by transistors Qi and Q₂ are applied to terminals 126 and

128, respectively, of transformer 120. As the second order harmonic distortion signals e provided by transistors Qi and Q₂ are in phase but applied to different terminals 126 and 128 of transformer 120 they are cancelled out such that the resulting second order harmonic distortion e in the output signal V_out is substantially zero. The same applies analogously to higher even-order harmonic distortions in case transistors

and Q₂ match precisely. LIST OF REFERENCE NUMERALS:

100 Electronic circuit 102 Control terminal 104 Drain terminal 106 Capacitor 108 Drain terminal 110 Control terminal 112 DC current source 114 Source terminal 116 Source terminal 118 Inductance 120 Transformer 122 Primary winding 124 Secondary winding 126 Terminal 128 Terminal

Claims

CLAIMS:

1. An electronic circuit for voltage to current amplification of a high frequency signal (V,„ι), the electronic circuit comprising: a first transistor (Qi) having a first control terminal (102) for applying the high frequency signal, a first source terminal (114), and a first drain terminal (104) of the first transistor being coupled to virtual ground, a second transistor (Q₂) having electrical properties being substantially equivalent to the first transistor, the second transistor having a second control terminal (110) being coupled to ground, a second source terminal (116) and a second drain terminal (108), an inductance (118) being coupled between the second drain terminal and a supply voltage, a current source (112) for providing a DC current, the current source being connected to the first and second source terminals.

2. The electronic circuit of claim 1 , wherein the first and second transistors are substantially identical field effect transistors.

3. The electronic circuit of claim 1 or 2, wherein a capacitor (106) is connected between the first drain terminal and ground for AC de-coupling of the first drain terminal, and the first drain terminal being coupled to the supply voltage.

4. The electronic circuit of claim 1, 2 or 3, wherein the current source is a high impedance current source.

5. The electronic circuit of any one of the preceding claims, wherein the second drain terminal (108) provides a non-inverted amplified output signal (V_out)-

6. The electronic circuit of any one of the preceding claims, wherein the first drain terminal (104) provides an inverted amplified output signal (V_out).

7. The electronic circuit of any one of the preceding claims 1 to 6, wherein the high frequency signal is an intermediate frequency (IF) signal.

8. The electronic circuit of any one of the preceding claims 1 to 7, wherein the high frequency signal is a television broadcast signal.

9. A cable television (CATV) system having at least one electronic circuit according to any one of the preceding claims 1 to 8.