CN111262568A - Quick photoelectric coupler capable of eliminating Miller effect and implementation method - Google Patents
Quick photoelectric coupler capable of eliminating Miller effect and implementation method Download PDFInfo
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- CN111262568A CN111262568A CN202010271567.8A CN202010271567A CN111262568A CN 111262568 A CN111262568 A CN 111262568A CN 202010271567 A CN202010271567 A CN 202010271567A CN 111262568 A CN111262568 A CN 111262568A
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Abstract
The invention discloses a quick photoelectric coupler capable of eliminating Miller capacitance influence, which comprises an optical coupling input circuit, a transistor photoelectric coupler circuit, an emitter follower base bias generation circuit, a bias stabilization circuit and a load resistor RLThe input signal passes through the optical coupler input circuit resistor RVEntering the transistor photocoupler circuit, the emitter follower circuit being formed by a transistor T2The internal transistor T of the transistor photocoupler circuit TIL1111Collector of the transistor T2The emitter and the 12V power supply are connected and operated sequentially through resistors R1 and R2 to form the emitter follower base bias voltage generating circuit, the connecting point of the resistors R1 and R2 is connected with the base of a transistor T2, a capacitor C1 forms the bias voltage stabilizing circuit, and the collector potential of the photoelectric tube is kept constant by adjusting the base bias voltage of T2, so that the Miller effect is eliminated.
Description
Technical Field
The invention relates to a technology for realizing a quick photoelectric coupler by eliminating Miller capacitance, wherein a common photoelectric coupler is easily influenced by the Miller effect due to the existence of the Miller capacitance, so that the timeliness of data transmission is poor, a photoelectric triode of the photoelectric coupler is integrated in a cascade circuit, so that a collector of a photoelectric tube has stable direct current potential, the voltage amplification factor of the photoelectric tube is almost zero, the Miller capacitance is eliminated, and the working frequency of the photoelectric coupler is accelerated.
Background
The photoelectric coupler is a group of devices which transmit electric signals by taking light (including visible light, infrared rays and the like) as a medium, and has the functions of isolating an input circuit and an output circuit at ordinary times and enabling the electric signals to pass through a transmission mode of an isolating layer when needed, so that the optical signals (a transmitting end) and the electric signals (a receiving end) are not interfered with each other, and the photoelectric coupler has good electric insulation capacity and anti-interference capacity.
The transistor output type optical coupler is the most common photoelectric coupler, the input end of the transistor output type optical coupler is of a direct current signal or alternating current signal control type, the output end of the transistor output type optical coupler is a transistor (a single body or Darlington, the latter has higher current transmission ratio), and the type of optical coupler is widely applied to various applications by virtue of the characteristics of low price and universality.
The transistor output optical coupler is characterized in that: the photoelectric receiver of the optical coupler uses a phototriode, so the defects are obvious: the transmission speed is slow, and the time sequence delay is large. The reason that the transmission speed of the transistor type photoelectric coupler is relatively slow is essentially that due to the existence of miller capacitance inside a transistor (including a field effect transistor), the miller effect causes the equivalent input capacitance to be increased, and the frequency characteristic to be reduced.
In theory, as long as the value of the miller capacitance is reduced, the influence of the miller effect is reduced, and the operating frequency of the transistor can be increased.
Disclosure of Invention
The invention aims to solve the technical problem of providing equipment which has simple structure, low manufacturing cost and reliable use and can quickly improve the working frequency of a common transistor photoelectric coupler.
In order to achieve the above object, the present invention provides a fast photoelectric coupler capable of eliminating the influence of miller capacitance, which is characterized in that: the rapid photoelectric coupler comprises a photoelectric coupling input circuit and a transistor photoelectric couplerCombiner circuit, emitter follower base bias generation circuit, bias stabilization circuit, and load resistor RLA circuit; the optical coupler input circuit is composed of a resistor RVThe input signal passes through a resistor RVEntering the transistor photocoupler circuit TIL111, the emitter follower circuit is composed of a transistor T2The internal transistor T of the transistor photocoupler circuit TIL1111Collector of the transistor T2Through the load resistor R, a 12V power supplyLCircuit connection transistor T2The 12V power supply is connected and operated through resistors R1 and R2 in sequence to form the emitter follower base bias generating circuit, the connection point of the resistors R1 and R2 is connected with the base of the transistor T2, the capacitor C1 forms the bias stabilizing circuit, the negative electrode of the capacitor C1 is grounded, and the positive electrode of the capacitor C1 is connected with the base of the transistor T2.
Drawings
Fig. 1, 2, and 3 are included to provide a further understanding of the present invention and form a part of the present application, and fig. 1 is an electrical schematic diagram of the present invention designed to reduce miller capacitance for fast operation of a transistor-type photocoupler. Fig. 2 is an output current-voltage characteristic of a transistor. FIG. 3 is an efficiency comparison diagram of a common optical coupler and a fast optical coupler during high-speed data transmission.
Detailed Description
1 miller capacitance and miller effect
The capacitance is not separated from the frequency, and the capacitance has the following relation with the frequency: a large capacitance responds poorly to high frequencies but well to low frequencies, while a small capacitance responds poorly to low frequencies and very well to high frequencies.
Generation of miller effect
A miller capacitance is a capacitance connected across the output and input of an amplifying device or circuit, and the effect of the miller capacitance on the frequency characteristics of the device or circuit is referred to as the miller effect.
The Miller effect is effected by amplifying the input capacitance, i.e. the Miller capacitance C enables the device or circuit to be of equal valueEffective input capacitance increase (1 + A)V) Where Av is the voltage gain of the circuit. Therefore, the frequency characteristics of the circuit device or the circuit can be greatly reduced due to the small miller capacitance.
Briefly: for electron tubes, the capacitance between the screen and the grid; for a transistor, the capacitance between the collector and the base is the barrier capacitance of the collector junction capacitance; for the field effect transistor, the capacitances between the drain and the gate are all miller capacitances.
When the transistors are used as common emitter mode or common source mode amplifying circuit, the phase of output end voltage is opposite to that of input end voltage, so that the charging and discharging current of the capacitor (such as collector junction capacitor) is increased, and the capacitor is seen from the input end of the device or circuit end as if the capacitor is increased by (1 + A)V) These phenomena are due to the miller effect.
Summarizing the above, for the common emitter amplifying circuit, the essence of the miller effect is caused by the phase inversion of the base potential and the collector output potential, thereby adding the collector junction capacitance CBCBetween voltage is enlarged by (1 + A)V) The current flowing through the junction capacitor becomes very large, namely the resistance of the collector junction becomes very small, so that the base input signal almost directly flows to the collector output end, and the common-emitter amplifying circuit does not have the amplifying function; however, the voltage at the two ends of the emitter junction is not in reverse phase, so the Miller effect does not exist, and the Miller effect does not exist in the common base amplifying circuit because the signals are output from the emitter input and the collector, and the input voltage and the output voltage are not in reverse phase relation.
Elimination of Miller capacitance
The effect of the miller capacitance can be properly reduced by using a technique such as a balancing method (or a neutralization method) in which a so-called neutralization capacitor is connected in parallel between the output end and the input end of the circuit, and the voltage on the neutralization capacitor is opposite in phase to the voltage on the miller capacitance, so that the current passing through the neutralization capacitor is exactly opposite in direction to the current passing through the miller capacitance, thereby achieving the purpose of mutual cancellation.
Therefore, in order to effectively suppress the miller effect, it is required that the neutralization capacitance and the miller capacitance are exactly matched, but in reality, since the output capacitance of the transistor as the miller capacitance is often voltage-dependent, it is difficult to completely match the neutralization capacitance and the miller capacitance, and thus various improvements are required.
Electrical principle for realizing rapid work of transistor type photoelectric coupler by reducing Miller capacitance
Photocouplers generally consist of three parts: the basic working principle of the photoelectric transmitting end, the photoelectric receiving end, the signal amplifying and shaping at the output end, the driving conversion circuit and the like can be described as follows: the input electric signal drives the light emitting source (including LED light emitting diodes or laser with various wavelengths), the other end of the isolated light emitting source is received by the optical detector (photoresistor, optical diode, phototriode, etc.) to generate current, the current is further amplified and then output, the conversion of 'electricity- > light- > electricity' is completed, and the functions of input, output and isolation are finally achieved.
In the conventional photocoupler on the market, the photo-receiving end is usually formed by a phototriode, and the operation modes of the phototriode all belong to a common emitter structure, so as mentioned above, although the photocoupler of the common emitter connection method has the advantage of excellent isolation between the transmitter and the receiver, the output of the phototriode is too slow when used for data communication, which is caused by the miller effect caused by the miller capacitance.
On the basis of keeping the advantage of good optical coupling isolation, in order to change the defect of poor frequency characteristic of the optical coupling and realize high-speed communication, the collector of the photoelectric receiving triode can be assumed to maintain a constant potential, so that the miller capacitance (base-collector capacitance) of the common emitter working photoelectric triode can not work, the influence of the miller effect is eliminated, the rapid work of the transistor type optical coupling is realized, and the electrical working principle of the transistor type photoelectric coupler which can realize the rapid work is shown in figure 1.
As can be seen from fig. 1, the fast photocoupler is regarded as a cascade amplification circuit as a whole, that is, the output of the primary photocoupler has been changed to the output of the cascade amplification circuit, and the cascade amplification circuit includes a photocoupler input circuit, a transistor photocoupler circuit, an emitter follower base bias generation circuit, a bias stabilization circuit, a load circuit, and the like.
In the cascade amplifying circuit of fig. 1, the primary phototransistor T1Has been integrated in the cascade amplifier circuit, the overall observation cascade amplifier circuit, the photoelectric tube T1Transistor T being in common emitter amplification mode and thus having Miller effect2For common base mode without Miller effect, the circuit signal is from T2The collector of the amplifier, so theoretically, the Miller effect exists in the cascade amplifying circuit.
Dynamic resistance of transistor
Photoelectric tube T1Since it is the common emitter mode of operation, T1The output current-voltage characteristic curve of (a) must also be as shown in fig. 2 below.
Horizontal axis in FIG. 2u CEIs a photoelectric tube T1The voltage drop between the collector and the emitter of (b),i Cis T1The output current-voltage characteristic curve of the field effect transistor is similar to that of fig. 2, so that the output current-voltage characteristic curves of the bipolar transistor (triode) and the field effect transistor are substantially horizontal, i.e. the output currenti CSubstantially independent of voltageu CEAnd is thus changed, so that the photocell T1Can be considered a constant current source.
The horizontal characteristic curve also shows the photoelectric tube T1The output ac resistance (i.e., dynamic resistance) of (a) is approximately infinite. In fact, the output current-voltage characteristic curve is not completely horizontal due to Early effect (for BJT, bipolar transistor), or due to channel length modulation effect, etc. (for FET, field effect transistor), but the dynamic resistance must be large.
The flatter the voltage-current characteristic curve of the component is, the smaller the ac conductance thereof is, and the larger the reciprocal of the ac conductance (dynamic conductance), i.e., the ac resistance, or called dynamic resistance, is.
Emitter follower circuit for realizing photoelectric tube T1Constant collector potential
As mentioned above, as long as the phototriode T can be realized1The collector potential of (2) is kept constant, i.e. T is realized1The Miller effect is eliminated, so that the transistor T needs to be analyzed2And a common base amplifier composed of a peripheral circuit for ensuring T1Is constant.
Photoelectric tube T1Is approximately infinite, as the transistor T2The emitter resistance of (1); resistance R1、R2The voltage divider circuit is composed of T2If the transistor T provides a bias voltage2Base current ofI B2Can be ignored for T2The analysis of (a) will be much simpler.
As long as the transistor T2Is selected to satisfyI R2＞＞I B2Then, thenI R1≈I R2WhereinI R1To pass through a resistor R1Current of，I R2To pass through a resistor R2Current of。At this time point B (resistance R)1、R2The crossing point of (see FIG. 1)
The above formula shows T2The base potential is almost determined only by the resistor R1、R2To pairV CCPartial pressure, independent of ambient temperature, i.e. when the temperature changesU BIs substantially unchanged.
However, the establishment of the above formula is conditional, i.e. when T2Emitter resistor of
I.e. by
Then, calculate T from above2The formula of the base potential is established, the specific sourceFor a matter of understanding, refer to relevant sections of teaching materials of analog electronic technology, and are not described herein again. WhereinR e2Is T2Of the emitter, i.e. the photocell T1The output dynamic resistance of (1); rb2=R1// R2(ii) a BETA is T2The direct current amplification factor of (1).
As can be seen from FIG. 1, T2The type is BC547B, and BETA thereof is 200-400; t is2The emitter resistance of is the photoelectric tube T1Almost infinite dynamic resistance of (a); according to the parameter shown in FIG. 1, Rb2=6.5//1=0.82kΩ。
So do all the aboveU BThe formula of (c) holds.
By calculatingU BThe formula can be calculated, the voltage divider R1、R2Make the transistor T2The base potential of the capacitor C is stably biased at 1.5V1The function of the transistor T is to ensure that the base bias voltage is basically stable and unchanged when the current rapidly fluctuates2The emitter junction voltage of (2) is kept constant at 0.8V.
By observing T1、T2In a connection mode of (1), standing at T2From the point of view of (1), T can be actually determined2As an emitter follower circuit, so that it is clear that the photocell T1The collector electrode(s) always maintain a constant (dc) voltage, then the photocell T1Voltage amplification factor A of circuit V0, thus making the photocell T1The miller capacitance (collector junction capacitance) does not function.
The reason why the above cascade circuit operates faster is that the collector voltage of the photo transistor can be kept constant, and therefore the influence of the miller capacitance inside the transistor can be disregarded, and as a result, the operating speed of the photo transistor can be made faster, as shown in fig. 3, which shows the comparison of the waveforms of the two circuits at 30kHz data transmission, the upper waveform being the conventional common circuit, and the lower waveform being the cascade amplifying circuit, that is, the fast photo coupler, indicating that the timeliness of the fast photo coupler is actually increased by a lot.
Matters of attention
The fast photocoupler has a disadvantage that its output signal level cannot drop to 0, but the optimum level may be 1, and the TTL integrated circuit has such a characteristic that the TTL operating voltage is only 12V.
The circuit can basically work in a 5V power supply, and only R needs to be changed appropriately1The resistance is of course better with CMOS integrated circuits.
When the circuit is verified through experiments, the maximum working current of the light-emitting tube in the photocoupler TIL111 (shown in FIG. 1) is not required to be exceeded by 100mA, and the maximum working current can be reduced by the voltage reduction resistor RVTo ensure that RVCan be calculated from the following formula:
input signal voltage in the formulaU inUnit is volt, luminous tube currentI LEDIn amperes.
Summarizing the above, the common emitter amplification circuit has poor frequency characteristics of the transistor type photoelectric coupler due to the existence of the miller capacitance, but the common base amplification circuit has no miller capacitance and has good frequency characteristics; the common base electrode amplifying circuit is used as a load of the photoelectric tube, the common base electrode amplifying circuit can be used as an emitter follower, and the collector potential of the photoelectric tube is kept constant by adjusting the base electrode bias voltage of the common base electrode amplifying circuit, so that the Miller effect is eliminated.
Claims (1)
1. A fast photoelectric coupler capable of eliminating the influence of Miller capacitance and an implementation method thereof are characterized in that: the rapid photoelectric coupler comprises a photoelectric coupling input circuit, a transistor photoelectric coupler circuit, an emitter follower base bias generation circuit, a bias stabilization circuit and a load resistance circuit; the optical coupler input circuit is composed of a resistor RVThe input signal passes through a resistor RVEntering the transistor photocoupler circuit TIL111, the emitter follower circuit is composed of a transistor T2The internal transistor T of the transistor photocoupler circuit TIL1111Collector of the transistor T2Through the load resistance circuit R, a 12V power supplyLConnecting transistor T2The 12V power supply is connected and operated through resistors R1 and R2 in turn to form the emitter follower base bias generating circuit, the connection point of the resistors R1 and R2 is connected with the base of the transistor T2, the electrolytic capacitor C1 forms the bias stabilizing circuit, the negative electrode of the capacitor C1 is grounded, and the positive electrode of the capacitor C1 is connected with the base of the transistor T2.
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2000354325A (en) * | 1999-06-10 | 2000-12-19 | Fuji Electric Co Ltd | Overload and short-circuit protection circuit of slave unit for interfacing actuator and sensor |
| CN204517789U (en) * | 2015-05-27 | 2015-07-29 | 沈震强 | Two-forty optocoupler integrated chip |
| CN211656113U (en) * | 2020-04-09 | 2020-10-09 | 山西工程技术学院 | Quick photoelectric coupler capable of eliminating influence of Miller capacitance |
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2020
- 2020-04-09 CN CN202010271567.8A patent/CN111262568A/en active Pending
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2000354325A (en) * | 1999-06-10 | 2000-12-19 | Fuji Electric Co Ltd | Overload and short-circuit protection circuit of slave unit for interfacing actuator and sensor |
| CN204517789U (en) * | 2015-05-27 | 2015-07-29 | 沈震强 | Two-forty optocoupler integrated chip |
| CN211656113U (en) * | 2020-04-09 | 2020-10-09 | 山西工程技术学院 | Quick photoelectric coupler capable of eliminating influence of Miller capacitance |
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