Abstract:
A field effect transistor circuit comprises a field effect transistor ( 1 ) having drain (D) and source (S) terminals for connection to respective power supply (Vss,  4 ) rails and a gate terminal (G) for receiving an input signal. The circuit further comprises a diode ( 6 ) which has its anode (B) connected to the gate terminal (G) of the transistor ( 1 ) and its cathode for connection to a bias voltage source ( 7 ), Vb). The diode ( 6 ) is arranged such that when the circuit is in use, the voltage level of the gate terminal (G) of the field effect transistor ( 1 ) is maintained at or below a predetermined value.

Description:
The present invention relates to low voltage transistor biasing, for example for use in a power amplifier. 
     DESCRIPTION OF THE RELATED ART 
     Field effect transistors (FETs) in power amplifiers can be used to control the gain of the amplifier by adjusting the gate-source voltage (V GS ) of the transistor. In some cases, for example, MESFET transistors, it is necessary to bias the gate of the transistor with a negative voltage. However, since the negative bias voltage has to be generated from a limited common positive supply voltage, like a battery, negative voltages of high magnitude are difficult to generate. 
     In some applications, for example, in radio telephones, it is necessary to be able to turn the transistor completely off so as to guarantee that very little radio frequency energy is transmitted when the phone is not supposed to transmit. 
     This can be accomplished by biasing the gate of the transistor to a DC level which ensures that the voltage at the gate never reaches above the turn-on voltage of the transistor. 
     The DC bias voltage determines how much of the AC signal is amplified, which therefore determines the gain of the amplifier in a large signal situation, such as a power amplifier. Thus, a large negative bias voltage will ensure that the transistor remaining in an off state. For example, for an input AC signal of ±1.5 V, and the turn-on voltage of the transistor being −1.5 V, the DC bias voltage would need to be −3 V or lower. 
     To achieve a DC bias voltage of −3 V from a common supply voltage of +3 V, previously used amplifiers have used a DC/DC converter. The most simple DC/DC converters (charge pumps) include a capacitor and a switch to produce a negative voltage. The switch gives some loss and a reasonable converted negative supply voltage which can be expected is thus −2.5 V. One way to improve this is to add another capacitor which is fed by the generated negative supply as ground, which gives a reasonable negative supply of −5 V. However, the extra components add cost, consume space, and are hard to integrate, since the capacitors have to be fairly large. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a field effect transistor circuit comprising a field effect transistor having drain and source terminals for connection to respective power supply rails and a gate terminal for receiving an input signal, the circuit further comprising a diode, having its anode connected to the gate terminal of the transistor and its cathode for connection to a bias voltage source, wherein the diode is arranged such that when the circuit is in use, the voltage level of the gate terminal of the field effect transistor is maintained at or below a predetermined value. 
     According to a second aspect of the present invention, there is provided a method of biasing a field effect transistor having drain and source terminals for connection to respective power supply rails and a gate terminal for receiving an input signal, the method comprising: 
     arranging a diode such that its anode is connected to the gate terminal of the transistor and its cathode is connected to a bias voltage source, the diode being arranged such that when the circuit is in use, the voltage level of the gate terminal of the field effect transistor is maintained at or below a predetermined value. 
     In a preferred embodiment, this allows the transistor to be readily turned off, using the available supply voltage, without requiring as many additional components as in the prior art circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a circuit diagram of a transistor circuit embodying the present invention; 
     FIG. 2 is a graph illustrating the voltage at the gate of the transistor of FIG. 1; 
     FIG. 3 is a circuit diagram of a second transistor circuit embodying the present invention; 
     FIG. 4 is a graph illustrating voltage levels produced in the circuit of FIG. 3; and 
     FIG. 5 shows results from a typical simulation of a circuit embodying the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a circuit diagram of a first embodiment of the present invention. A field effect transistor (FET)  1 , having gate (G), drain (D) and source (S) terminals is connected between a supply voltage (VSS) (drain D) and ground  4  (source S). The circuit has an AC signal input  2 , and a signal output  3 . Output impedance  5  is provided at the drain terminal D. The circuit of the present invention is of particular use when it is necessary to bias the gate of the transistor  1  with a negative gate-source voltage, for example when the transistor is a metal-semiconductor FET (MESFET). 
     In order to bias the gate terminal G of the transistor  1 , a diode  6  is connected between the gate G and a bias voltage terminal  7  (V b ) 
     When the diode  6  is conducting the maximum voltage at point B of the circuit equals V b +V t , where V t  is the threshold voltage of the diode (for example 0.7 V). The maximum voltage at the gate G of the transistor is thus limited to V b +V t  V. 
     The maximum peak voltage of the DC biased AC signal at the gate G is therefore limited to this maximum voltage level, as illustrated in FIG.  2 . Consequently, the AC signal floats to find its own mean level such that the peak level V b +V t  does not exceed the maximum allowed level. 
     Therefore, if the gate voltage V G  is to be maintained at a level which ensures that the transistor remains OFF, the maximum voltage (V G ) must be below the turn-on voltage (V TON ) of the transistor: 
     
       
         V G ≦V TON   (Eq. 1) 
       
     
     Thus, 
     
       
         V b +V t ≦V TON   (Eq. 2) 
       
     
     At the limit of the transistor switching ON, 
     
       
         V b +V t =V TON   (Eq. 3) 
       
     
     For example, for a diode having a threshold voltage of 0.7 V, and a transistor having a turn-on voltage of −1.5 V, the external bias voltage (V b ) must be −2.2 V or lower. This level is readily achieved by a simple DC/DC converter, in contrast to the prior art devices which required an external voltage of −3 V. 
     The gate voltage can be represented as a DC signal which has the input AC component superimposed on it: 
     
       
         V G =V GDC +V AC   (Eq. 4) 
       
     
     Therefore the DC bias voltage level (V GDC ) for the gate voltage is set by the external bias voltage V b , the diode threshold voltage V t  and the AC signal peak voltage V AC , according to the following relationship, which is clear from FIG.  2 : 
     
       
         V GDC =V b +V t −V AC   (Eq. 5) 
       
     
     Thus it can be seen that for a transistor having a turn on voltage (V TON ) of −1.5 V, expecting an AC signal input having a peak voltage (V AC ) of 1.5 V, and having the diode threshold voltage and external voltage as before (0.7 V and −2.2V respectively), a DC bias voltage (V DC ) of −3 V is produced. This is in line with the requirement that was described previously in order to prevent the transistor being turned on. 
     However, if the AC peak voltage swing is small, the circuit of FIG. 1 will not successfully bias the transistor because the diode  6  will not turn on. In order to overcome this problem, an impedance transformation circuit can be used. 
     One circuit embodying the present invention in which such a transformation is made is shown in FIG. 3. A three pole low pass filter  8  is provided by two inductors  81  and  82 , and a capacitor  83 . The AC input signal is supplied at the input terminal  2 , through an input impedance  9 . As an example, an input signal of 7 dBm and an input impedance of 50 Ω will give an AC voltage signal of 0.7 V peak to peak. 
     The low pass filter components serve to produce a larger voltage swing (V c ) across the capacitor than is input to the filter  8  or output therefrom. For example, if the impedance (achieved by the inductor and capacitor) at the capacitor  83  is 200 ohms, then a 7 dBm signal will give an AC voltage swing of 1.4 V peak to peak across the capacitor  83 . Thus, the voltage across the diode  6  is large enough to turn the diode ON, even though the input AC signal has a low peak level. The voltage levels at various points of the circuit of FIG. 3 are illustrated in FIG.  4 . 
     The components  81 ,  82 , and  83  are chosen in dependence upon the level of input AC signal level that can be expected. The values are chosen so that the voltage swing is of sufficient size to turn on the diode  6 , for a given input power. 
     For example, for a 900 MHz input signal, inductors  81  and  82  may be 20 nH each, and capacitor  83  may be 2.6 pF, for a gate voltage V AC  of 0.7V peak (=7 dBm), and a transistor turn on voltage of −1.5V. In general, conventional circuit simulations packages can be used to choose the values of the components. FIG. 5 shows a typical output from a circuit simulation package. 
     The DC bias required at the gate G of the transistor is determined by the peak level of the AC component of the gate voltage. In order for the transistor to remain off: 
     
       
         V GAC +V GDC ≦V TON   (Eq. 6) 
       
     
     where V GAC  is the AC component peak voltage, V GDC  is the DC bias level, and V TON  is the turn on voltage of the transistor. 
     The external bias voltage V b  is then determined from equation 5 above, with V AC  being the converted voltage signal V C  across the capacitor  83 . 
     For example, for V C  of 1.4 V, V GAC  of 0.7 V peak (1.4 V peak to peak), a diode threshold voltage of 0.7 V, and a transistor turn on voltage of −1.5 V, the desired DC bias level at the gate is −2.2 V (V TON −V GAC ) This means that the external bias voltage V b  must be less than −1.5 V. 
     This magnitude of negative voltage is significantly less than the previously required level, and is thus more easily produced. 
     The diode  6  can be positioned anywhere along the input line, between the input line and the bias voltage Vb, but is most effective where the voltage swing is largest. 
     It will be appreciated that embodiments of the present invention allow a smaller magnitude negative bias voltage (V b ) to be used. Such a voltage is relatively simple to generate.