JPS643365B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a feedback amplifier in which a positive feedback loop is provided within a negative feedback loop to reduce distortion without impairing the stability of the entire amplifier. By detecting the occurrence of clipping and providing a bypass circuit in the positive feedback loop, stable operation can be achieved even during clipping.

Figure 1 shows a feedback amplifier previously invented by the applicant, where 1 is an amplification stage with a transfer function A ₁ , 2 is an amplification stage with a transfer function A ₂ , 3 is a negative feedback circuit with a transfer function β, and 4 is an amplification stage with a transfer function A 2. is a positive feedback circuit with a transfer function γ.

In this way, a positive feedback loop is provided in the negative feedback loop by the amplification stage 1 with the transfer function A ₁ and the positive feedback circuit 4 with the transfer function γ, and the round transfer function A ₁ ·γ of this positive feedback loop is expressed as A ₁・γ=K/(1+S・T) [However, 0<K<
2, S is a complex frequency, and T is a time constant] By setting, the gain of the amplification stage constituted by the amplification stage 1 and the positive feedback circuit 4 can be increased. In other words, if the gain of the amplification stage composed of the amplification stage 1 and the positive feedback circuit 4 is _A1 ', then _A1 '= _A1 /1- _A1・γ= _A1 /1-K/(1+S・T). . Therefore, if we include the condition 0<K<2, |1-K/(1+S・T)|<1, and the gain A ₁ ' of the amplification stage composed of the amplification stage 1 and the positive feedback circuit 4 is positive feedback. It can be seen that A ₁ ′>A ₁ compared to the gain A ₁ of the amplifier stage 1 when the circuit 4 is not provided. Furthermore, if the distortion component generated at the input of the subsequent amplification stage 2 is d, and the distortion output component outputted by this distortion component d in the configuration shown in FIG. 1 is S _d1 , then S _d1 is S It is expressed as _d1 =A ₂ /1+A ₁ ′・A ₂・β・d. On the other hand, if the distortion output component in the configuration without the positive feedback circuit 4 is S _d2 , then S _d2 becomes S _d2 =A ₂ /1+A ₁ ·A ₂ ·β ·d. Since A ₁ ′＞A ₁ , S _d1 <S _d2 ,
A positive feedback loop is provided in the negative feedback loop _by a transfer function A ₁ , an amplification stage ₁ , and a positive feedback circuit with a transfer function γ. 1+S・T)
[However, by setting 0<K<2], distortion can be effectively removed.

The stability of the system at this time can be determined by looking at the phase of the round transfer function of the negative feedback loop at a gain of 1 in FIG. That is, the loop transfer function of the negative feedback loop is stable if the gain is 1 and the phase is not delayed by 180 degrees or more. The round transfer function of the negative feedback loop in Fig. 1 is A ₁ /1 - A ₁ · γ · A ₂ · β. Here, from the system in Fig. 1, the positive feedback circuit 4
If we consider the open-circuit transfer function of the negative feedback loop when is removed, we get A ₁・A ₂・β. Figure 2 shows an example of the frequency vs. gain and phase characteristics diagram of the transfer function of A ₁ , A ₂ , and β. Figure 2 shows the characteristics of the open loop transfer functions A ₁ , A ₂ , and β of a sufficiently stable negative feedback system.

Here, the open-circuit transfer function A ₁ ·γ of the positive feedback loop of the amplifier stage ₁ and the positive feedback circuit ₄ in FIG.・Assume that the transfer function of A ₂・β is a sufficiently stable negative feedback system as shown in Figure 2, and furthermore, for the angular frequency 1/T ₀ where A ₁・A ₂・β=1, By setting the transfer function of A ₁・γ so that /T is sufficiently small, the open-loop transfer function A ₁・A ₂・β/(1−A ₁・γ) of the negative feedback system in Figure 1 can be obtained.
has frequency vs. gain and phase characteristics as shown in FIG. Looking at Figure 3, A ₁・A ₂・β/(1−
The angular frequency at which A ₁・γ)=1 is shown in Figure 2.
It is the same as the angular frequency 1/T ₀ where A ₁・A ₂・β=1, and the phase delay at this time is also less than 180°, and the first
It can be seen that the system in the figure is stable.

However, in the amplifier configured as described above, if the output voltage is clipped by the power supply voltage, A ₂ =0, and the negative feedback loop becomes inactive. At this time, if the amplification stage 1 is not clipped, only the positive feedback loop system composed of the amplification stage 1 and the positive feedback circuit 4 remains active.
In the system shown in Figure 1, the stability of the system is achieved as described above by providing a positive feedback loop within a stable negative feedback loop.The negative feedback loop is removed, and the amplifier stage 1 If only the positive feedback loop constituted by the positive feedback circuit 4 and the positive feedback circuit 4 remains, it becomes unstable when the DC gain of the positive feedback loop is 1 or more. Therefore,
With the open loop transfer function A ₁ · γ of the positive feedback loop composed of the amplification stage 1 and the positive feedback circuit 4 set to A ₁ · γ = K / (1 + S · T) [K≧1], the first If the output voltage of the amplifier shown in the figure is clipped by the power supply voltage, there is a problem that the amplifier shown in FIG. 1 becomes unstable.

The present invention provides a feedback amplifier that solves these problems.

An embodiment of the present invention will be described below with reference to FIG.

In Fig. 4, 5 is an input terminal, 6 and 7 are transistors that constitute the first differential amplification stage, 8 and 9 are transistors that constitute the current mirror load, 10 is an emitter follower, and 11 and 12 are current detection transistors. A transistor and a diode constitute a current mirror circuit, 13 is a voltage amplification stage transistor, 14 is a bias circuit, 15 is an SEPP output stage, 16 is an output terminal, 17, 18, and 19 constitute a negative feedback circuit 3 in FIG. Resistors and capacitors, 20 and 21 constitute the positive feedback circuit 4 of FIG. 3 together with the transistor 11 and its collector resistor 22, 23 is the emitter resistance of the transistor 13, 24 and 25 are phase compensation resistors, and capacitors, 26 and 27 are power supply terminals, and 28 is a diode connected between the collector of the transistor 11 and the collector of the transistor 13; this diode 28 detects that the output voltage is clipped by the power supply voltage; It has the function of forming a bypass circuit in the feedback loop.

In the above configuration, the collector potential of the transistor 13 is V ₁₃ , the positive power supply voltage is +B, the voltage across the resistor 23 when clipped is V ₂₃ , and the transistor 13
The collector-emitter voltage when clipping is V _13sat.
Then, the output voltage clips when V ₁₃ =+B-V ₂₃ -V _13sat .

And the collector voltage of transistor 11 is
If V ₁₁ and the voltage across the resistor 22 are V ₂₂ , then V ₁₁ = +B - V ₂₂ , so if V ₂₂ is set a little larger than V ₂₃ + V _13sat. , the diode 2
8 is in cut-off, and diode 28 turns on just before the output voltage clips.

Therefore, current begins to flow from the collector of transistor 13 to the collector of transistor 11, forming a bypass circuit in the positive feedback circuit. Therefore, the load on the transistor 11 becomes zero, that is, the positive feedback loop transfer function K becomes zero, and γ in FIG. 1 becomes zero. In this way, the positive feedback loop is opened just before the output voltage is clipped, thus solving the problem of system instability during clipping.

Note that although the diode 28 in FIG. 4 is effective only for the positive side clip, it can be constructed in the same way for the negative side as well.

As described above, the present invention provides a positive feedback loop within the negative feedback loop, and uses this positive feedback loop to increase the gain of the preceding amplification stage and reduce the distortion generated in the subsequent amplification stage. Since the feedback amplifier opens the positive feedback loop by detecting that the output voltage is clipped by the power supply voltage, the amplifier can operate stably in all operating conditions.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an amplifier to which the present invention is applied, FIGS. 2 and 3 are diagrams explaining its operation, and FIG. 4 is a circuit diagram of an embodiment of the present invention. 6, 7... First differential amplifier stage, 10... Emitter follower, 11, 20, 21, 22... Positive feedback circuit, 13... Voltage amplifier stage transistor, 15...
...SEPP output stage, 17, 18, 19... Negative feedback circuit, 28... Clip detection diode.

Claims (1)

[Claims] 1. A negative feedback loop, which is provided within the negative feedback loop, and has a circular transfer function of K/(1+S·T) [where 0<K<2, S is a complex frequency and T is a time constant] A feedback amplifier comprising a positive feedback loop represented by , and a bypass circuit that detects when the output voltage is clipped by the power supply voltage and sets K of the open-loop transfer function to zero during the clipping period.