JPH0440109A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To prevent oscillation from generating even when an input signal is enlarged and a feedback gain is reduced by forming a load part to decide the non-feedback gain of an amplifying part as a variable impedance element so that the impedance can be small when the input signal is large and the impedance can be large when the input signal is small. CONSTITUTION:In the variable gain amplifier having a dynamic range equipped with a negative feedback means 1 to enlarge the gain when the input signal is small and to reduce the gain when the input signal is large, the load part to decide the non-feedback gain of an amplifying part 2 is formed with a variable impedance element 3 so that the impedance can be small when the input signal is large and the impedance can be large when the input signal is small. Namely, inversely connected diodes D1 and D2 are parallelly connected to a resistor R1 for deciding the non-feedback gain, and a bias V1 is loaded by using diodes D3-D5 so that potentials can almost be equal at both ends of the diodes D1 and D2 when there is no input signal. Thus, since the gain is reduced by the negative feedback means 1 when the input signal is large, the non- feedback gain is reduced as well, and no oscillation is generated.

Description

[Detailed Description of the Invention] [Summary] This invention relates to a variable gain amplifier with a wide dynamic range that has a negative feedback means that increases the gain when the input signal is small and decreases the gain when the input signal is large. The aim is to provide a variable gain amplifier with a wide dynamic range that does not cause oscillation even when the size is reduced.
The load portion that determines the non-feedback gain of the amplifier section is configured as a variable impedance element that has a small impedance when the input signal is large and a large impedance when the input signal is small.

[Industrial application field]

The present invention relates to an improvement in a variable gain amplifier with a wide dynamic range, such as a preamplifier in a receiving section of optical fiber communication.

FIG. 4 is a block diagram of an optical receiver having a preamplifier to which the present invention is applied. In FIG. 4, an input optical signal is converted into an electrical signal by an avalanche photodiode APD, and a current is - Voltage conversion is performed and amplified, the main amplifier 11 amplifies the signal, the waveform equalizer 12 performs waveform equalization, and the signal is input to the discriminator 16.

A timing extraction circuit 13 extracts a timing signal from the waveform-equalized signal, a resonator 14 selects only the timing signal, a timing signal amplifier 15 amplifies the signal, and inputs the signal to a discriminator 16. 1.0 is identified and the identified signal is output.

In this case, a wide dynamic range is required for the optical input signal, and the characteristics of the dynamic range are approximately determined by the preamplifier 10.

Therefore, the preamplifier 10 needs to have a wide dynamic range, and a variable gain amplifier with a wide dynamic range is used.

[Conventional technology]

FIG. 5 is a circuit diagram of a conventional variable gain amplifier with a wide dynamic range, and FIG. 6 is a gain frequency characteristic diagram of the circuit shown in FIG.

The circuit shown in Fig. 5 is a variable gain amplifier with a wide dynamic range used as a preamplifier, and the transistor Q2 is an emitter follower circuit, and the current input to the transistor Q1 from the avalanche photodiode APD via the capacitor C is 13 , the current amplification factor of the transistor Q1 is hfe, and the load resistance is R1, the non-feedback gain G0 is approximately 1, expressed as XhfeXR1, and the gain frequency characteristics are as shown in FIG. 6, 20.

The negative feedback circuit includes a diode Dfl, in parallel with the resistor Rf.
Create a configuration in which Df2 are connected in reverse to each other, and when the input signal is small, the impedance is increased, and when the input signal is large, the impedance is decreased, and the gain/% of the variable gain amplifier is 30.31 in Figure 6. As shown, when the input signal is small, it is large, and when the input signal is large, it is small, to widen the dynamic range.

[Problem to be solved by the invention]

However, in order to widen the dynamic range, when the input signal is large and the feedback gain is made small, the amount of feedback becomes large as shown in Figure 6, and the second-order slope of the non-feedback gain (the phase rotation is twice the first-order slope). 22 (rotates up to 180 degrees), and there is a problem that oscillation may occur.

An object of the present invention is to provide a variable gain amplifier with a wide dynamic range that does not cause oscillation even when the input signal is large and the feedback gain is reduced.

[Means to solve the problem]

FIG. 1 is a block diagram of the principle of the present invention.

As shown in Figure 1 (when the input signal is small, the gain is increased,
In a variable gain amplifier with a wide dynamic range that has a negative feedback means 1 that reduces the gain when the input signal is large, the load part that determines the non-feedback gain of the amplifier section 2 has a small impedance when the input signal is large, and a When the impedance is small, the variable impedance element 3 has a large impedance.

[For production]

According to the present invention, when the input signal becomes large, the negative feedback means 1
Although the gain becomes small at , the impedance of the variable impedance element 3 that determines the non-feedback gain also becomes small, and the non-feedback gain becomes small.

Therefore, the amount of negative feedback does not increase and the gain decreases, so oscillation no longer occurs.

〔Example〕

FIG. 2 is a circuit diagram of a variable gain amplifier with a wide dynamic range according to an embodiment of the present invention, and FIG. 3 is a gain frequency characteristic diagram of the circuit of FIG. 2.

The difference in Fig. 2 from the conventional example shown in Fig. 5 is that diodes D1 and D2 are connected in parallel with the resistor R1 that determines the non-feedback gain, and when there is no input signal, the diodes Di and D2 are connected in parallel. The difference is that the bias V1 is applied using the diodes D3 to D5 so that the potentials at both ends are approximately equal, so this difference will be mainly explained.

The diode D5 is inserted to raise the potential of the transistor Q1 so that when the diodes Dfl and Df2 are turned on, a voltage is also applied to the point A of the emitter of the transistor Q2. The potential at the four points on the base of Q2 is the potential of three diodes, so the potential v2 at the four points and the potential of the diode D1. A diode D3. D4 is connected in series.

In this way, even if the input signal is small and the voltage v2 changes,
When IVI-V21 is smaller than the ON voltage of DI and D2, the diode DI.

Since the impedance of D2 is large, the non-feedback gain G is approximately I s Xhf eXRl.

The input signal increases, the voltage v2 changes, and Vl-V2
When + exceeds the on-voltage of the diodes DI and D2, the impedance of the diodes Di and D2 becomes small.

Therefore, the non-feedback gain G0 when Vl-V2>on voltage
is approximately x, xhfex (internal resistance of DI), and V2
-Vl>ON voltage, the non-feedback gain G0 is approximately I,
xhf eX (internal resistance of D2), and the non-feedback gain G becomes small.

Figure 3 shows the gain state in this case. When the input signal is small, both the non-feedback gain and the feedback gain are large, and when the input signal becomes large, the non-feedback gain and the feedback gain are both small. The amount of feedback does not change much, and the phase is 1.
Do not use the secondary tilted part that rotates 80 degrees (therefore no oscillation will occur).

〔Effect of the invention〕

As explained in detail above, according to the present invention, when the input signal is small, both the non-feedback gain and the feedback gain are large, and when the input signal becomes large, both the non-feedback gain and the feedback gain become small, so the amount of feedback does not change much. This has the effect of eliminating oscillation.

[Brief explanation of drawings]

Figure 1 is a block diagram of the principle of the present invention. Figure 2 is a circuit diagram of a variable gain amplifier with a wide dynamic range according to an embodiment of the present invention. Figure 3 is a gain frequency characteristic diagram of the circuit in Figure 2. Figure 4 5 is a circuit diagram of a conventional variable gain amplifier with a wide dynamic range, and FIG. 6 is a gain frequency characteristic diagram of the circuit shown in FIG. 5. It is. In the figure, 1 is a negative feedback means, 2 is an amplifier, 3 is a variable impedance element, 10 is a preamplifier, 11 is a main amplifier, C is a capacitor, 14 is a resonator, 15 is a timing signal amplifier, and 16 is a discriminator.吏APD is an avalanche photodiode, Ql, Q2 are transistors, R1-R3, Rf are resistors.忰り 13 Iju V Fudanrou/) Actual I row n Tynamy/7 range/) Wide-
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Claims (1)

[Claims] In a variable gain amplifier having a negative feedback means (1) that increases the gain when the input signal level is low and decreases the gain when the input signal level is high, the load part that determines the non-feedback gain of the amplifier section (2) is connected to the input signal. A variable gain amplifier with a wide dynamic range, characterized in that the variable impedance element (3) has a small impedance when the input signal is large, and a variable impedance element (3) whose impedance becomes large when the input signal is small.