JPH0452652B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a time constant circuit that combines a switched capacitor and a capacitor.

The audio circuits of magnetic recording and reproducing devices such as VTRs and tape recorders are equipped with equalizer circuits that correct the frequency characteristics of the audio head and flatten the reproduction frequency characteristics. Figure 1 shows the playback equalizer characteristics of a VTR audio circuit, and Figure 2 shows a time constant circuit consisting of resistors 2 and 3 and capacitor 4 that realizes this.If terminal 5 is input and terminal 6 is output, the input and The transfer function representing the frequency characteristics of the output is the resistance 2,
3. Let the values of capacitance 4 be R ₁ , R ₂ , C, and the angular frequency ω
Then, R ₂ +1/jwc/R ₁ + (R ₂ +1/jwc) = R ₂ /R ₁
+R ₂ ·1+1/jwcR ₂ /1+1/jwc (R ₁ +R ₂ ) (1). Therefore, the high-frequency cutoff frequency is 1/2πcR ₂ and the low-frequency cutoff frequency is 1/2πc(R ₁ +R ₂ ), so if each constant is selected appropriately, the characteristics shown in Figure 1 can be obtained. It is possible.

Here, when considering that Figure 2 is built into an IC, the cutoff frequency is 50Hz and 1.33kHz as shown in Figure 1.
In order to achieve this, the values of the time constants c (R ₁ +R ₂ ) and cR ₂ are large, and it was considered impossible to achieve this with an economical IC chip size. In other words, in conventional IC processes, capacitors occupy a larger chip area than other elements, so capacitors exceeding approximately 200 PF were treated as external components of the IC. this
Even if 200PF were built in, to obtain a low cutoff frequency of 50Hz, (R ₁ + R ₂ ) would be 16MΩ, and to obtain a high cutoff frequency of 1.33KHz, the value of R ₂ would be 600KΩ.
In other words, R ₁ is 15.4MΩ and R ₂ is 0.6MΩ, which are unrealizable values. Furthermore, even if this high resistance were achieved, variations in resistance and capacitance would be completely independent, so it would be impossible to incorporate a highly accurate time constant air path as shown in Figure 2. It was used as a part.

Therefore, the inventors replaced the resistors 2 and 3 in FIG. 2 with an equivalent resistance (hereinafter abbreviated as a switched capacitor) realized by a switch 7 operated by a clock signal 13 and a capacitor 14 as shown in FIG. I thought about that.

In FIG. 3, the opening and closing of switch 7 is controlled by a clock signal 13. First, connect switch 7 to contact 1
0, connect to terminal 8 side, connect to capacitor 14, terminal 8
Instantly charges the voltage. Next, connect switch 7 to contact 1
1, connect it to the terminal 9 side, and discharge the charge in the capacitor 14.

Here, if the voltages at terminals 8 and 9 are V ₁ , V ₂ , clock period T, frequency c, and capacitance value C, then the capacitance 1
The charge of the capacitor 14 when the other end of the capacitor 14 is connected to the terminal 8 is CV ₁ , and the charge of the capacitor 14 when the other end of the capacitor 14 is connected to the terminal 9 by the switch 7 is CV ₂ , so the unit Since the amount of charge movement per time, that is, the current I, is expressed as I=C/T(V ₁ -V ₂ ), the relationship between terminals 8 and 9 is equivalently Req=V ₁ -V ₂ /I This means that a resistance expressed as =1/Cc (2) has been inserted. Here, the equivalent resistance Req is the reciprocal of the product of the clock frequency c and the capacitance C, so the smaller the capacitance, the higher the resistance can be achieved, and when integrated into an IC, the chip occupies a smaller area. For example, clock frequency 100KHz, capacity
If it is 1PF, it will be 10MΩ.

As mentioned above, a switched capacitor consisting of a capacitor and a switch can provide high resistance with a small capacitance as long as the clock frequency is selected appropriately.If this is combined with a capacitor, a relatively large time constant can be obtained with a small capacitance. If it can be obtained and the capacitance ratio accuracy is ensured, the accuracy of the time constant will also be improved. In FIG. 4, the resistors 2 and 3 of FIG. 2 are replaced with the switched capacitor 12 of FIG. 3. In FIG. Here, the second and third
Components with the same functions as those in the figure are given the same reference numerals. Here, the resistor 2 in FIG. 2 is the switch 17.
The switched capacitor 15 consisting of the switch 18 and the capacitor 19 and the resistor 3 correspond to the switched capacitor 1 consisting of the switch 18 and the capacitor 20.
It is 6. The inverter 22 is an inverter for making the switches 17 and 18 operate in opposite phases to each other with respect to the clock signal 13. 21 is a hold capacitor to prevent the terminal 6 from being open when the switches 17 and 18 are connected as shown by the solid line, and is a small capacitor that is not affected by the frequency characteristics with respect to the signal frequency to be handled. good.

With the above configuration, in principle, the second
This is an equivalent circuit in which the resistors 2 and 3 shown in the figure are replaced with switched capacitors. That is, if the clock frequency c and the values of capacitances 19 and 20 are set to C ₁ and C ₂ ,
From the second equation, R ₁ =1/C ₁ c, R ₂ =1/C ₂ c. Therefore, since equation (1) can be expressed as equation (3), R ₂ /R ₁ +R ₂・1+1/jwcR ₂ /1+1/jwc(
R ₁ + R ₂ )=C ₁ /C ₁ +C ₂・1fc/jw C ₂ /C/1+1/jw
/c (C/C ₁ +C/C ₂ ) (3) Once the clock frequency c is determined, the high cutoff frequency is
Since the low band frequency is determined by cC ₂ /C and c/(C/C ₁ +C/C ₂ ), the frequency characteristics are determined by the capacitance values of capacitors 19 and 20 relative to the capacitance value of capacitor 4. For example, if the clock frequency is 100KHz, the low range in Figure 1,
C ₁ that satisfies the high cutoff frequency, 50Hz, 1.33KHz,
Let's find the values of C ₂ and C. From the frequency ratio, 1.33KHz/50Hz = 1/C・cC ₂ /C (1/C ₁ c + 1/C
₂ c)=(1+C ₂ /C ₁ ).

The capacitance ratio C ₂ /C ₁ = 25.6. Minimum capacity C ₁ to 1PF
Then, C ₂ =25.6PF. Further, the value of C is 306PF from 2π×50Hz=100KHz/C/0.5P+C/12.6, taking into account the low cutoff frequency of 50Hz.

As described above, in principle, Figure 2 can be completely replaced with a switched capacitor as shown in Figure 3, and as long as the capacitance ratio is accurate, the high and low cutoff frequencies can be determined with sufficient accuracy. can do. However, there are many problems when converting these into ICs. First, to realize an equalizer circuit for the audio circuit of a VTR or tape recorder, a large capacitance ratio is required, which not only makes it difficult to achieve high precision, but also requires a large built-in capacity. do not become. In other words, in the configuration shown in Figure 4, even if the minimum capacity of 26 is 1PF, C ₂ is
25.6PF, C must be 306PF, and the total capacitance is 332.6PF. Normally, MOS capacitors are used for these, but 1PF is roughly equivalent to one transistor, so the area occupied by the capacitance on the chip is It will be extremely large. Furthermore, the capacity ratio is 1:20 ~
If it is about 30, it is not difficult to achieve an accuracy of about 5%, but achieving a capacity ratio of about 300 is nearly impossible in terms of accuracy.

Second, in an IC, a parasitic capacitance is added to one of the terminals due to the structure of the capacitor, so FIG. 4 is not necessarily equivalent to FIG. 2. That is, as shown in FIG. 5, the structure of the MOS capacitor in the IC consists of a P-type substrate 32, an N-type epitaxial layer 33, a P-type diffusion layer 34, an Al wiring 35, a gate oxide film 36, and an oxide film 37. configured, terminal 3
A capacitor 41 is formed between 8 and 39. Here, the P-type substrate is usually connected to the lowest potential of the IC, and the N-type epitaxial layer is connected to the highest potential. Due to the structure shown in Figure 5, the P-type diffusion layer and the N-type epitaxial layer are P-
An NB junction is formed, and a junction capacitance 40 is formed parasitically, and the equivalent circuit becomes as shown in FIG. The value of this parasitic junction capacitance 40 is not negligible compared to the value of the target capacitance 41. Furthermore, the value changes depending on the voltage applied to the junction, which causes distortion. Therefore, the capacity 4,1
If the aluminum side (gate oxide film side) is grounded without grounding the junction side shown in FIG. , the signal applied to terminal 5 becomes a distorted signal at terminal 6. Further, the capacitance must be designed taking this parasitic capacitance into consideration, but it is difficult to obtain a highly accurate capacitance ratio since it has dependence on applied voltage. In this case, capacity 1
There is no problem if the junction side of the capacitor structure shown in FIG. 5, which constitutes capacitors 9, 20, and 21, is grounded, but as for capacitor 4, the connection does not necessarily provide the desired characteristics. That is, if the Al side of the capacitor 4 is set to the terminal 6 side and the junction side is set to the switch 18 side, the parasitic capacitance 26 can be removed, but a parasitic capacitance 27 is added instead.
Similarly, FIG. 4 is not only not equivalent to FIG. 2, but also has the problem of distortion due to the parasitic capacitance 27.

As described above, when converting a time constant circuit consisting of a resistor and a capacitor into a switched capacitor, simply replacing the resistor with a switched capacitor is not very practical.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and provide a highly accurate time constant circuit using a switched capacitor.

The main purpose of the present invention is to replace the resistor constituting the time constant circuit with an equivalent resistor realized by a switch operated by a clock signal and a capacitor, and improve the time constant accuracy by using the Mira capacitance effect. The purpose of this invention is to clarify the method of connecting the components of a time constant circuit, and to provide a time constant circuit that can be built into an IC with an economical chip size. That is, in the present invention,
In a time constant circuit consisting of at least two resistors and one capacitor, the two resistors are switched
The capacitor is constituted by a Mira capacitor circuit consisting of an amplifier with a substantially constant gain and a capacitor inserted between the input and output of the amplifier.

FIG. 7 is a principle block diagram showing an embodiment of the present invention, and parts having the same functions as those in FIG. 4 are given the same reference numerals. Here, in order to avoid the effects of the various parasitic capacitances mentioned above, connect the junction sides of the capacitors 19, 20, 21, and 27 of the configuration shown in FIGS. 5 and 6 to the ground or to the lower impedance as shown in the figure. , the parasitic amounts 23, 24, 25, and 28 are equivalently removed.

In addition, in order to realize a highly accurate equalizer circuit with a built-in IC, the insertion points and connection points of the capacitor 4 and the switched capacitor 16 in FIG. 4 have been changed to make it easier to realize a Mira capacitor circuit. Furthermore, when considering the conversion of switched capacitors to ICs,
Since a direct-coupled circuit is common, it is necessary to consider the processing of the signal superimposed on the direct current, so a signal 31 is superimposed on the direct current power supply 32. This power supply has a capacity of 27
It also serves as a bias power supply when the amplifier 29 constituting the Mira capacitor is configured as a differential amplifier. In such a configuration, the capacitance value of capacitance 27
C ₃ , if the amplification degree of the amplifier 29 is A, then the terminal 3
The capacitance value C ₄ (1+
A) It becomes C ₃ . Therefore, as mentioned above, in order to realize the equalizer characteristics shown in Figure 1, the capacitances C ₁ and C ₂ must be
The capacitance ratio of equivalent capacitance C ₄ at terminal 30 must be C ₁ :C ₂ :C ₄ =1:25.6:306, but since C ₄ is a Mira capacitance, the actual value of capacitance 27 C ₃ may be 1/(1+A) of the required capacity. In other words, C ₁ : C ₂ : C ₄ = C ₁ : C ₂ : (1+A) C ₃ = 1:25.6: (1+A) 306/1+A, so the capacitance ratio is reduced according to the amplification degree of the amplifier 29. If the amplification degree can be set accurately, the capacitance ratio can be highly accurate.
For example, if the amplification degree A is 9 times, the actual capacity is
The capacitance ratio of C ₁ :C ₂ :C ₃ can be 1:25.6:30.6. With this level of capacitance ratio, it is easy to set the capacitance ratio to ±5% as described above. Also, the amplifier 19
The degree of amplification can generally be determined by the resistance ratio of the resistors that make up the amplifier, and its accuracy is 5% if the resistance ratio is about 20.
It is equally easy to make it within 1:25.6:
The capacitance ratio accuracy of 306 can be set to the worst 10%.

Furthermore, in an IC, the area occupied by a capacitor on a chip is larger than that of elements such as resistors and transistors, and approximately 1 PF is equivalent to one other analog element.・If realized with a capacitor, even if the minimum capacity is 1PF, the maximum capacity, 306PF, is equivalent to 306 elements, which is not economical. However, if a Mira capacitor is used, the capacitance value can be reduced to one factor of the amplification degree. Since a normal operational amplifier can be realized with a maximum of 20 elements, it is better in terms of chip area to configure Mira capacitors in the chip area as well.

For example, if the amplification degree is 9, _C3 needs only 30.6PF, and amplification can be achieved with about 20 elements, so 1PF
If this is converted into one element, 306 elements can be reduced to about 50 elements.

Here, when using Mira capacitance in a circuit using a switched capacitor, some measures are required. First, in order to make the capacitance 27 and the amplifier 29 have accurate Mira capacitance values, the influence of the parasitic capacitance 28 must be eliminated. For this purpose, the junction side of the capacitor 27 may be connected to the output side of the amplifier 29, and the amplifier 29 may be configured to be capable of sufficiently driving this capacitor.

Second, the amplifier 29 must have an extremely high input impedance and a sufficiently large output dynamic range relative to the signal level input to the terminal 30. Thirdly, when the switch 18 is connected as shown in the broken line, the amplifier 29 has a capacitance of 27
Since full feedback is achieved through the oscillation, the amplifier must be oscillated-free even at this time.

FIG. 8 shows an amplifier for realizing Mira capacitance that satisfies the first, second, and third conditions described above.

First, in order to achieve the first condition of ultra-high input impedance, a MOS-FER 35 is installed at the input. In order to increase the second condition, the dynamic range of the output,
The DC power supply 32 shown in FIG. A bias voltage is applied to the output terminal 53. In such a configuration, if the values of the resistors 47 and 48 and the resistors 49 and 50 are the same, the MOSFETs 35 and 36 will also be the same, and the DC base voltages of the transistors 41 and 42 will also be the same, so that the current flows to the transistors 41 and 42. The currents are also equal. If the resistors 46 and 47 are also set to the same value, the same current as that flowing through the diode-connected PNP transistor 44 will flow through the PNP transistor 43, and the supply current of the PNP transistor 43 and the sink current of the transistor 41 will eventually become equal, so the DC power supply Since there is no current flowing in or out from terminal 32, there is no DC voltage drop across resistor 51.
The DC voltage No. 3 becomes the voltage of the DC power supply 32. Therefore, since the resistor 51 exhibits a current change corresponding to the change in the signal input to the terminal 30, the voltage of the DC power supply 32 is reduced to 1/2 of the voltage of the power supply terminal 34.
If you choose it, it will be the biggest dynamic cleanse. Furthermore, in this configuration, it is said that the frequency characteristics are not sufficient in the normal IC process.
Since the PNP transistor is not used as an amplification element, there is no oscillation due to full feedback.

If you only want to achieve Mira capacitance, there is no need to pay special attention to the amplifier, but in order to achieve Mira capacitance in a switched capacitor circuit,
As shown in the present invention, special measures are required.

As described above, according to the present invention, it is possible to incorporate a large time constant circuit that handles audio frequencies into an IC, and moreover, it can be realized with high accuracy with an economical chip size.

[Brief explanation of the drawing]

Figure 1 is a reproduction equalizer characteristic diagram of a VTR audio circuit, Figure 2 is a diagram showing a time constant circuit to realize an equalizer, Figure 3 is a principle diagram of a switched capacitor, and Figure 4 is the resistance of Figure 2. Figure 5 shows a circuit in which MOS is replaced with a switched capacitor.
A structural diagram of the capacitor, Figure 6 is a diagram showing its equivalent circuit,
FIG. 7 is a diagram showing a specific embodiment of the present invention, and FIG.
The figure is a diagram showing an amplifier for realizing the Mira capacitance of the present invention. 1: Equalizer/frequency characteristics, 2, 3: Resistance,
4: Capacity, 5, 6: Terminal, 7: Switch, 12,
15, 16, 18: switched capacitor, 13: clock signal, 27: capacitor, 29: amplifier.

Claims (1)

[Claims] 1. Two switched capacitors, one end of which is grounded and the other end of which is switched between a first terminal and a second terminal by a switch operated by a clock signal, the first switched capacitor being・The first terminal of the capacitor is used as the input terminal, and the second terminal is used as the second switched terminal.
Connect it to the first terminal of the capacitor as an output terminal,
and connecting to the second terminal of the second switched capacitor a Mira capacitor circuit consisting of an amplifier with a substantially constant gain and a capacitor inserted between the input and output terminals of the amplifier; The above capacitor constituting the second switched capacitor is a MOS capacitor, the junction side of the MOS capacitor is grounded, the gate oxide film side is the other end, and the capacitor of the Mira capacitor circuit is the MOS capacitor. , applicable
The gate oxide film side of the MOS capacitor is used as the input terminal side of the amplifier, and the junction side is used as the output terminal side. , the sources of which are connected to a first constant current source, and the connection point between the source of the first MOS-FET and the first constant current source is connected to a first NPN type transistor constituting a first common emitter amplifier. The collector of the first transistor is connected to the output terminal of the amplifier, the collector of the second transistor, and a reference voltage source via a resistor, and the collector of the first transistor is connected to the base of the transistor of the second MOS-FET. A gate of the second MOS-FET is connected to the reference voltage source, a drain of the second MOS-FET is connected to the power supply terminal, and a source of the second MOS-FET is connected to the second constant current source. The connection point with the constant current source No. 2 is connected to the base of a third NPN type transistor constituting the second common emitter amplifier, and the collector of the third transistor is connected to the second constant current source.
A time constant circuit characterized in that the time constant circuit is connected to the collector of a fourth transistor forming a current mirror circuit together with the transistor.