JPS6148288B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a feedback circuit configured by cascading a plurality of field effect transistors.

Conventionally, a feedback circuit is a so-called inverter circuit (C -MOS amplifier circuit) and this inverter circuit (C-
A multi-stage amplifier circuit is formed by cascading multiple stages of MOS amplifier circuits, an input signal is applied to the gate input terminal of the first stage inverter circuit via an input impedance element, and an output is output from the drain electrode of the last stage inverter circuit. It was constructed by taking out the inverter and feeding it back to the gate input terminal of the first stage inverter circuit via a feedback impedance element. In other words, since the voltage gain per stage of a normal C-MOS amplifier circuit is approximately 10 to 20 times (usually channel length 7 to 8 μ), this inverter circuit is required to obtain an amplifier circuit with high voltage gain. had to be used in multi-stage cascade connection. However, since C-MOS circuits require a relatively large area, using a large number of inverter circuits occupies a large area on the semiconductor substrate, resulting in high prices, which is extremely disadvantageous economically. . Furthermore, when a negative feedback circuit is formed using such an amplifier circuit, the feedback factor becomes large and self-sustained oscillation occurs in a high frequency band. Therefore, in order to suppress self-oscillation in the high frequency band, it is possible to give the feedback circuit a frequency characteristic to reduce the output gain at high frequencies. However, in such a case, a large capacitance element is required as the feedback impedance element, which is difficult to integrate on one integrated circuit board, and when it is externally attached, the cost increases accordingly.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a low-cost feedback circuit.

In order to achieve the above object, the feedback circuit of the present invention is connected in cascade to the output stage with a length longer than the channel length between the source and drain of each FET of the C-MOSFET forming the last stage inverter circuit as the output stage. It is characterized by increasing the channel length of each FET of at least one C-MOSFET in the other stages.

Hereinafter, one embodiment of the feedback circuit of the present invention will be described in detail using a negative feedback circuit with reference to the drawings.

FIG. 1 is a circuit diagram showing this embodiment. In addition, in order to simplify the explanation, in this example, C--
An example of cascading three stages of MOS inverter circuits was presented.

From the figure, the negative feedback circuit of this embodiment connects the gate electrodes of the P-channel MOSFET and N-channel MOSFET in common and uses this as the input terminal, and connects the drain electrodes in common and uses this as the output terminal. A three-stage cascade type amplifier circuit in which the input terminals and output terminals of C-MOS inverter circuits 3, 4, and 5 are connected to each other and the source electrodes are connected to the power supply (V _DD ) and ground, and the first stage inverter circuit. A resistor R _i as an input impedance element Z _i between the gate electrode of No. 3 and input terminal 1, and the first stage inverter circuit 3
The feedback element Z _f is provided in the feedback path between the gate electrode (input stage) of the inverter circuit and the drain electrode (output stage) of the last stage inverter circuit, and is composed of a series connection means of a resistor R _f and a capacitor C _f . be done.
Note that the channel lengths of the first and second stage C-MOSFETs (approximately 15 μm or more) are set longer than the channel lengths of the third stage C-MOSFETs (approximately 7 to 8 μm). In this way, by increasing the channel length between the race and drain of the first and second stage MOSFETs, the saturation resistance of the channel r _ds (=ΔV _DS /ΔI _DS ;V
_DS ≧V _G −V _T ) increases, and the influence of the depletion layer due to fluctuations in the source-drain voltage also decreases.
Therefore, C- formed with a long channel length.
The voltage gain per stage of the MOS inverter amplifier circuit increases as the length of the channel increases, as shown in FIG.

FIG. 2 is a diagram showing changes in channel length L [μ] and accompanying changes in voltage gain in an amplifier circuit configured with one stage of C-MOS inverter circuits. In addition, the P channel that constitutes the inverter circuit
The channel lengths of the FET and N-channel FET are made equal.

As is clear from the figure, the channel length is 10μ.
When , the voltage gain is about 13 times, when it is 20μ, it is about 85 times, and when it is 30μ
It is about 150 times when it is , and about 170 times when it is 60μ. Therefore, as in this embodiment, the amplifier circuit has a long channel length (approximately 15μ or more, in some cases 30μ, 50μ or more).
By using MOSFETs, it is possible to obtain an amplifier circuit with a high voltage gain with a small number of connected stages.

Furthermore, the longer the channel length, the lower the high frequency characteristics of the circuit, so self-oscillation in the high frequency band can be prevented even in high voltage gain operation, and the feedback impedance element C _f can be omitted.

Here, if an amplifier circuit is formed using MOSFETs with a long channel length, the voltage gain will increase, but the mutual conductance G _n will decrease, so the output impedance will increase and the feedback voltage will fluctuate, making it difficult to obtain a good feedback circuit. becomes difficult. However, as in this example, the C-MOS located at the last stage
By shortening the channel length of the inverter circuit, mutual conductance can be increased, output impedance can be lowered, and a negative feedback circuit with high voltage gain and less feedback output voltage fluctuation can be obtained.

As described above, according to the present invention, the channel length of the C-MOS inverter circuit constituting the amplifier circuit can be increased, the saturation resistance r _ds can be increased, and high voltage gain can be obtained with a small number of connection stages. By making the channel length of the output stage (last stage) inverter circuit shorter than other channel lengths, we can create a stable negative feedback circuit with low output impedance and less feedback voltage fluctuation. Obtainable.

Although this embodiment has presented an example in which three stages of amplifier circuits are applied to the negative feedback circuit, the number of connected stages can be set arbitrarily. In addition, at least one inverter circuit having a channel length longer than that of the last stage inverter circuit is used.
The effects of the present invention can be obtained if the number of inverter circuits is greater than 1, and the channel length of each inverter circuit may be set freely under this configuration. Furthermore, as an amplifier circuit, C-MOSFET other junction type is used.
FETs may also be used. Further, although a negative feedback circuit is presented in this embodiment, a positive feedback circuit can also be created with the configuration of the present invention. Furthermore, when a negative feedback circuit is formed, one end can be grounded as an impedance element (resistance R _i ) in the input stage. However, the other end can also be connected to the gate of the first stage CMOSFET.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a negative feedback circuit showing an embodiment of the present invention, and FIG. 2 is a diagram showing the relationship between channel length and voltage gain of a C-MOS inverter amplifier circuit per stage. 1...Input terminal, 2...Output terminal, 3, 4, 5
...C-MOS inverter circuit, Z _i ...input impedance element, Z _f ...feedback impedance element, R _i , R _f ...resistor, C _f ... capacitor.

Claims (1)

[Claims] 1. In a feedback circuit in which a plurality of stages of complementary circuits including P-type and N-type field effect transistors are continuously connected, and feedback is provided between the input part of the complementary circuit in the first stage and the output part in the last stage, A feedback circuit characterized in that the channel length of the field effect transistor located in one stage is shorter than the channel length of any of the field effect transistors constituting other stages.