WO2007077409A1

WO2007077409A1 - An active load pull amplifier

Info

Publication number: WO2007077409A1
Application number: PCT/GB2006/000038
Authority: WO
Inventors: John David Rhodes
Original assignee: Filtronic Plc
Priority date: 2006-01-05
Filing date: 2006-01-05
Publication date: 2007-07-12

Abstract

An active load pull amplifier for amplifying a signal (V) of fundamental frequency (fo) comprising a main amplifier (2); an auxiliary amplifier (3), a 3dB coupled line coupler (9) comprising first and second inputs and first and second outputs; the outputs from the main (2) and auxiliary (3) amplifiers being connected to the first and second inputs of the coupled line coupler (9) respectively; and the 3dB coupled line coupler (9) being a quarter wavelength coupler at frequency fo.

Description

An Active Load Pull Amplifier

The present invention relates to an active load pull amplifier. More particularly, but not exclusively, the present invention relates to an active load pull amplifier having main and auxiliary amplifiers connected to a 3dB coupled line coupler, the main and auxiliary amplifiers being adapted to provide currents to the coupler in the ratio

Imain = a (1+x) -l≤X≤l

~^∞ _iγ O≤x≤l

= 2 ^JX

= 0 -l≤x≤O

with α being a positive constant, and (1+x) being proportional to the input voltage.

The first active load pull networks were incorporated in Doherty amplifiers, which are known.. Such amplifiers comprise main and auxiliary amplifiers the outputs of which are connected together by a transmission line which is a quarter wavelength long at the fundamental frequency of the signal being amplified. Such a transmission line however acts as a 1:1 or 1:-1 transformer at even harmonics resulting in their even harmonic voltages being connected directly between the main and auxiliary amplifiers. One consequence of this is that the signal from the main amplifier when operated in class B or class AB with significant harmonic content can cause the voltage on the auxiliary amplifier to become negative during an RF cycle. If the voltage becomes significantly negative this causes a high current in the auxiliary amplifier sufficiently large to destroy the output transistor.

The active load pull amplifier according to the invention seeks to overcome this problem.

Accordingly in a first aspect the present invention provides an active load pull amplifier for amplifying a signal V of fundamental frequency fo comprising

a main amplifier;

an auxiliary amplifier;

a 3dB coupled line coupler comprising first and second inputs and first and second outputs;

the outputs from the main and auxiliary amplifiers being connected to the first and second inputs of the coupled line coupler respectively; and

the 3dB coupled line coupler being a quarter wavelength coupler at frequency fo.

Preferably, the main and auxiliary amplifiers are connected to first and second voltage sources respectively, the voltage sources and amplifiers being adapted such that the output currents provided by the amplifiers are in relation

Imain = 0.(1+*) -l≤X≤l — ix 0 < x < 1 laux = 2

= 0 -l≤x≤O

The active load pull amplifier can comprise a signal splitter, the signal splitter having an input port for receiving an input voltage and first and second output ports connected to the inputs of the main and auxiliary amplifiers respectively, the splitter being for transmitting portions of input voltage to the main and auxiliary amplifiers respectively;

the splitter and main and auxiliary amplifiers being arranged such that the output currents provided by the main and auxiliary amplifiers are in the relation

Imain = OC(I+*) -l≤X≤l

- OC

-JX O≤x≤l

= 0 -l≤x≤O

Preferably, the active load pull amplifier can further comprise transformers between the outputs of the main and auxiliary amplifiers and the inputs of the coupled line coupler.

One output of the coupled line coupler can be connected to a load and the other output can be connected to a reactive termination. The reactive termination can be open circuit .

The active load pull amplifier can further comprise quarter wavelength transmission lines between the outputs of the main and auxiliary amplifiers and the inputs of the 3dB coupled line coupler, the reactive termination comprising a short circuit to ground.

The active load pull amplifier can further comprise non- integer quarter wave transmission lines between the outputs of the main and auxiliary amplifiers and the inputs of the 3dB coupler.

The splitter can introduce a 270° phase shift between the signals to the main and auxiliary amplifiers.

In a further aspect of the invention there is provided a multistage active load pull amplifier for amplifying a signal V of fundamental frequency fo comprising

first and second 3dB coupled line couplers each having two inputs and two outputs;

first main and first auxiliary amplifiers the outputs of which are connected to the first and second inputs of the first 3dB coupled line coupler;

second main and second auxiliary amplifiers the outputs of which are connected to the first and second inputs of the second 3dB coupled line coupler;

a third 3dB coupled line coupler having two inputs and two outputs, one of the outputs of the first 3dB coupler being connected to the first input of the third 3dB coupler, one of the outputs of the second 3dB coupler being connected to the second input of the third 3dB coupler;

Preferably the multistage active load pull amplifier further comprises a signal splitter, the signal splitter having an input port for receiving an input voltage and a plurality of output ports at least one output port connected to each of the inputs of the main and auxiliary amplifiers, the signal splitter and main and auxiliary amplifiers being arranged such that the output currents provided by the main and auxiliary amplifiers are in the relation

- o

The multistage active load pull amplifier can comprise a plurality of voltage sources, each of the inputs of the main and auxiliary amplifiers being connected to a voltage source .

At least one of the voltage sources can comprise a signal splitter for splitting the output of the voltage source between the inputs of a plurality of the main and auxiliary amplifiers.

Preferably the voltage sources and main and auxiliary amplifiers are arranged such that the currents provided by the main and auxiliary amplifiers are in the relation

= o -i < x < -y₂

One output of the third 3dB coupled line coupler can be connected to a load and the other output is connected to a reactive termination.

The reactive termination can be a short circuit to ground.

Wherein the reactive terminator can be open circuit, the connections between the first and second 3dB coupled line couplers and the third 3dB coupler each including a quarter wavelength transmission line.

The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which

Figure 1 shows in schematic form a known Doherty amplifier;

Figure 2 shows the equivalent network circuit for figure 1;

Figure 3 shows the relationship between the current provided by the main and auxiliary amplifiers of the circuit of figure 1;

Figure 4 shows the output voltage across the main and auxiliary amplifiers as a function of input voltage;

Figure 5 shows the efficiency of the amplifier of Figure 1 as a function of input voltage; Figure 6 shows in schematic form an active load pull amplifier according to the invention;

Figures 7 and 8 show outputs of the 3dB coupled line coupler for various input voltages at the two inputs;

Figures 9 to 11 show equivalent circuits for the coupled line coupler with R_opt being the optimum load resistance;

Figures 12 and 13 show an alternative embodiment of the active load pull amplifier according to the invention and the network equivalent circuit thereof;

Figure 14 shows a multistage active load pull amplifier according to the invention;

Figure 15 shows the relationship between the current provided by the first main and auxiliary amplifiers and input voltage;

Figure 16 shows the relationship between the current provided by the second main and auxiliary amplifiers and input voltage; and,

Figure 17 shows the efficiency of the amplifier of figure 14 as a function of input voltage .

Shown in Figure 1, in schematic form is a known Doherty amplifier 1. The Doherty amplifier 1 comprises main and auxiliary amplifiers 2,3, the outputs of which are connected to a load 4 as shown. The output of the auxiliary amplifier 3 is connected to the load 4 through a transmission line 5 which is adapted to be a quarter wavelength transmission line at the fundamental frequency of the signal to the amplified. The inputs of the amplifiers 2,3 are connected to the signal source 6 by a phase splitter 7 which introduces in 90° phase difference to the signals supplied to the main and auxiliary- amplifiers 2,3 with the signal to the auxiliary amplifier leading 3 to the signal to the main amplifier 2 by 90°.

The network equivalent circuit of figure 1 is shown in figure 2. The phase splitter provides 7 voltages to the amplifiers 2,3 such that the output current are in the relation

(where j indicates a 90° phase change) in response to an input voltage

as shown in figure 3.

From the circuit shown in Fig. 2, the impedances Z_τ and Z₂ are:

If the amplifiers are operated in Class B with shunt resonators to ensure sinusoidal voltages the R_opt normally- chosen to be:

and hence efficiency μ is:

π (l+x)² -_^ ._Λ μ=—τ -x 0<Λ:<1

4 (l+3x)

and is simply: π _n , -l≤x≤O μ =-(l+x)

This is shown in Fig. 5.

Thus, for amplitude modulation, the efficiency is the

TC maximum of — at both at full power and 6dB lower power

27T and has a minimum value in that range of — . At lower

9 signal levels where efficiency is not as important, the efficiency deteriorates proportionately to changes in x.

In the above analysis of the known Doherty circuit 1, it is assumed that resonant circuits are used at the output of the amplifiers 2,3 such that in the Class B configuration, the output voltages are constrained to contain only fundamental voltages. Using large microwave transistors for high power amplifiers, it is difficult to achieve high Q shunt resonators at the output of the devices. Thus, under the Class B operation where significant currents are generated at even harmonics, even, harmonic voltages propagate through the transmission line 5 which is quarter wavelength long at the fundamental. The transmission line 5 acts as a 1:1 or 1:-1 transformer at even harmonics resulting in these voltages being connected directly between the main and auxiliary amplifiers 2,3. One consequence of this is that the signal from the main amplifier 2 can cause the voltage on the auxiliary amplifier 3 to become negative during an RF cycle. If the voltage becomes significantly negative, this will cause a high current in the auxiliary amplifier 3 sufficiently large to destroy the output transistor.

Shown in figure 6 is an active load pull amplifier 8 according to the invention. The amplifier 8 is similar to that of figure 1 except the quarter wavelength transmission line 5 is replaced by a 3dB coupled line coupler 9. The coupled line coupler 9 is a quarter wavelength coupler at the fundamental frequency. The amplifier 8 also comprises a electronic splitter 10 adapted such that in combination with the amplifier the current supplied by the amplifiers 2,3 in response to the input voltage are in the relation

J-main — α(l + x ) - i ≤ x ≤ :

- CDC O ≤ x ≤ l

■J-aux ⁼ 2 ^J

0 - I ≤ Λ ≤ O

One embodiment of such an electronic splitter 10 is a 270° phase splitter, rather than a 90° phase splitter 7 as in known Doherty amplifiers 1. This can also be described as the output of the auxiliary amplifier 3 being opposite in sign to the output of the auxiliary amplifier 3 of known Doherty amplifiers 1 as described with reference to figure 1. In an alternative embodiment the electronic splitter 10 is a simple splitter and the required extra phase change and amplitude shaping is provided by the amplifiers 2,3. Any combination of splitter and amplifiers which provide the required extra phase change is considered to be according to the invention. Such am amplifier 8 acts as a normal Doherty amplifier 1 at the fundamental but the main and auxiliary amplifiers 2,3 are isolated from each other at all even harmonics . For the avoidance of doubt any electronic circuit which splits an input voltage and provides it to the main and auxiliary amplifiers 2,3 could be considered to be a splitter 10.

In an alternative embodiment of the invention the amplifier 8 lacks the splitter 10 for splitting a single input voltage between the main and auxiliary amplifiers 2,3. Separate voltage signals from separate sources are applied to the main and auxiliary amplifiers 2,3. These separate voltages are applied in the correct relationship to .produce the desired current relationship between outputs of the main and auxiliary amplifiers 2,3.

The operation of such a circuit can be understood with reference to figures 7 to 11.

Shown in figure 7 in schematic form is the 3dB coupled line coupler 9 of figure 6. This is constructed such that each line is a quarter of a wavelength long at the fundamental frequency and is designed such that if any three ports are terminated with a resistance load R_opt the input impedance at the fourth port is R_opt-

If ports 2, 3, 4 are terminated in R_opt/ then a voltage V at the fundamental frequency when applied to port 1 results in voltages

as shown in figure 7.

If in addition a voltage JV is applied to port 2, then

as shown in figure 8.

If a voltage V₂ containing only even harmonics V_2H is applied to port 2 then

V₂ = 0, V₃ = 0 V₄ = ±V_2H V_2H = even harmonic voltage

Similarly, if even harmonic voltages are applied to port 2 all of the signal appears at port 3.

In both cases the power at port 3 at the fundamental frequency is zero and hence any load may be removed and replaced by an open circuit. Under this condition analysis of the 3dB hybrid leads to a three port structure with an equivalent circuit shown in figure 9.

This circuit can be transformed to the circuit shown in figure 10. If port 4 is now terminated in R_opt and the impedance level scaled the equivalent circuit is simplified to the circuit shown in figure 11. This is identical to the equivalent circuit for the known Doherty amplifier 1 as shown in figure 2 apart from the 1:-1 transformer at port 2. Accordingly, by changing the sign of the signal in the auxiliary amplifier 3 and hence changing the relationship between I_mai_n and I_aux as explained above the circuit performs exactly as a known Doherty amplifier 1 at the fundamental frequency whilst ensuring that the signals at all even harmonics in the main amplifier 2 are isolated from these in the auxiliary amplifier 3.

In an alternative embodiment of the invention port 3 of the coupler is short circuited. Quarter wavelengths of transmission lines are included after both main and auxiliary amplifiers 2,3 prior to the connection to the 3dB hybrid as shown in figure 12.

At the fundamental frequency this reduces to the circuit shown in figure 13. This is the dual of the Doherty circuit and achieves the same load pull performance as the original circuit.

For large transistors in the amplifiers 2,3, the direct output impedances are very low compared to the normal 50Ω outputs. Thus, impedance transformers (not shown) have to be incorporated between the amplifiers 2,3 and the 3dB hybrid 9 which is normally realised in a 50 Ω impedance environment. These transformer networks have to be carefully designed to ensure that the required bandwidth is maintained under the dynamic load conditions as the signal varies in amplitude. When achieved, this will allow the full efficiency enhancing performance of the 3dB hybrid active load-pull system to be maintained over the desired bandwidth without the adverse effects which are generated by the even harmonic signals in the main and auxiliary amplifiers 2,3. Further increases in efficiency can be achieved if more than two amplifiers are used. Shown in figure 14 is a multistage active load pull amplifier 11 according to the invention. The amplifier 11 comprises first and second 3dB coupled line couplers 12,13 each having two input and two output ports . Connected to the two inputs of the first 3dB coupled line coupler 12 are first main and auxiliary" amplifiers 14,15. Connected to the inputs of the second 3dB coupled line coupler 13 are second main and auxiliary amplifiers 16,17.

One output from each of the first and second 3dB couplers 12,13 is connected to the input of a third 3dB coupled line coupler 18 via quarter wavelength transmission lines 19,20. The remaining outputs are open circuit. One of the outputs of the third 3dB coupled line coupler 18 is open circuit and the remaining output is connected to a load R_Opt 4.

The main and auxiliary amplifiers 14-17 are each connected to separate voltage sources, (not shown) . The voltage sources and amplifiers 14-17 are arranged such that the current supplied by the main and auxiliary amplifiers 14-17 as a function of input voltage are

Imain (l) = ( l÷ x ) -l ≤ X ≤ l

O ≤ x ≤ l

Iaux (l) = -j α*

= 0 - l ≤ x ≤ O

Imain (2) = "OCX O ≤ X ≤ l

as shown in figures 15 and 16.

In the range the efficiency o am lifier I_main(i)

increases linearly to the maximum value Fo

amplifiers Imain(1) and Imain(2) act as an active load pull pair via the output 3dB coupler with efficiency reducing

to — before — is reached at x =0. For O≤x≤l the 9 4 amplifiers I_mai_n(i) and I_auχ(i) act as an active load pull pair as do amplifiers I_main(2) and I_aux(2) and they combine with equal powers in the final output 3dB coupler 18. The equiripple efficiency is shown in figure 17.

A further embodiment of the invention is shown in figure

18. In this embodiment a splitter 21 receives an input signal and splits it between the four main and auxiliary amplifiers 14-17. In this embodiment the splitter 21 and amplifiers 14-17 are adapted so that the amplifiers 14-17 provide current in the correct ratio as described above.

A further embodiment of the invention is shown in figure

19. This embodiment is similar to that of figure 14 but without the quarter wavelength transmission lines between the outputs of the first and second 3dB couplers 12,13 and the inputs of the third 3dB coupled line coupler 18. In this embodiment the remaining output of the third 3dB coupled line coupler is connected to ground.

Further amplifier stages may be used if required although the increase in efficiency would be minimal compared to the four amplifier case.

Claims

1. An active load pull amplifier for amplifying a signal V of fundamental frequency fo comprising

a main amplifier;

an auxiliary amplifier;

2. An active load pull amplifier as claimed in claim 1, the main and auxiliary amplifiers being connected to first and second voltage sources respectively, the voltage sources and amplifiers being adapted such that the output currents provided by the amplifiers are in relation

Imain = 0.(1+*) -l≤X≤l

— ήc O≤x≤l laux = 2

= 0 -l≤x≤O

3. An active load pull amplifier as claimed in claim 1 further comprising a signal splitter, the signal splitter having an input port for receiving an input voltage and first and second output ports connected to the inputs of the main and auxiliary amplifiers respectively, the splitter being for transmitting portions of input voltage to the main and auxiliary amplifiers respectively;

Imain = C.(l+x) -l≤X≤l

lfE,-_r O≤x≤l

= 2 ^J

= 0 -l≤x≤O

4. An active load pull amplifier as claimed in any one of claims 1 to 3 , further comprising transformers between the outputs of the main and auxiliary amplifiers and the inputs of the coupled line coupler.

5. An active load pull amplifier as claimed in any one of claims 1 to 4 , where one output of the coupled line coupler is connected to a load and the other output is connected to a reactive termination.

6. An active load pull amplifier as claimed in claim 5, wherein the reactive termination is open circuit.

7. An active load pull amplifier as claimed in claim 5, further comprising quarter wavelength transmission lines between the outputs of the main and auxiliary- amplifiers and the inputs of the 3dB coupled line coupler, the reactive termination comprising a short circuit to ground.

8. An active load pull amplifier as claimed in claim 5, further comprising non-integer quarter wave transmission lines between the outputs of the main and auxiliary amplifiers and the inputs of the 3dB coupler.

9. An active loan pull amplifier as claimed in any one of claims 2 to 8, wherein the splitter introduces a 270° phase shift between the signals to the main and auxiliary amplifiers.

10. A multistage active load pull amplifier for amplifying a signal V of fundamental frequency fo comprising

11. A multistage active load pull amplifier as claimed in claim 10, further comprising a signal splitter, the signal splitter having an input port for receiving an input voltage and a plurality of output ports at least one output port connected to each of the inputs of the main and auxiliary amplifiers, the signal splitter and main and auxiliary amplifiers being arranged such that the output currents provided by the main and auxiliary amplifiers are in the relation

OC

Imain(l) (i+x) -l≤x≤l

O≤x≤l laux(l) = -J ax

= 0 -l≤x≤O

Imain(2) = "«* O≤X≤l

= 0 -l≤x≤O Iaux(2) = ²^ (1+Λ) O≤X≤l

= O -l<x≤-V₂

12. A multistage active load pull amplifier as claimed in claim 10, comprising a plurality of voltage sources, each of the inputs of the main and auxiliary amplifiers being connected to a voltage source.

13. A multistage active load pull amplifier as claimed in claim 12, wherein at least one of the voltage sources comprises a signal splitter for splitting the output of the voltage source between the inputs of a plurality of the main and auxiliary amplifiers.

14. A multistage active load pull amplifier as claimed in either of claims 12 or 13, wherein the voltage sources and main and auxiliary amplifiers are arranged such that the currents provided by the main and auxiliary amplifiers are in the relation

Imain(l) = — (1+ x) -l≤X≤l

O≤x≤l

Iaux(l) = -J O. X

= 0 -l≤x≤O

Imain(2) = -V-X O≤X≤l = 0 - l ≤ x ≤ O

iaux{2) = ¹^r- ( ^{1+ χ}) O ≤ x ≤ l

- /OC

(1+2JC) -y₂≤x<o

15. A multistage active load pull amplifier as claimed in any one of claims 10 to 14, wherein one output of the third 3dB coupled line coupler is connected to a load and the other output is connected to a reactive termination.

16. A multistage active load amplifier as claimed in claim 15, wherein the reactive termination is a short circuit to ground.

17. A multistage active load pull amplifier as claimed in claim 15, wherein the reactive terminator is open circuit, the connections between the first and second 3dB coupled line couplers and the third 3dB coupler each including a quarter wavelength transmission 1ine .

18. An active load pull amplifier substantially as hereinbefore described.

19. A multistage active load pull amplifier substantially as hereinbefore described.