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MC1495 Datenblatt(PDF) 9 Page - ON Semiconductor |
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MC1495 Datenblatt(HTML) 9 Page - ON Semiconductor |
9 / 16 page MC1495 http://onsemi.com 9 To set currents I3 and I13 to the desired value, it is only necessary to connect a resistor between Pin 13 and ground, and between Pin 3 and ground. From the schematic shown in Figure 3, it can be seen that the resistor values necessary are given by: R3 + 500 Ω = I3 |V–| –0.7 V R13 + 500 Ω = I13 |V–| –0.7 V Let V– = –15 V, then R13 + 500 = 14.3 V 1.0 mA or R13 = 13.8 k Ω Let R13 = 12 kΩ. Similarly, R3 = 13.8 kΩ, let R3 = 15 kΩ However, for applications which require an accurate scale factor, the adjustment of R3 and consequently, I3, offers a convenient method of making a final trim of the scale factor. For this reason, as shown in Figure 21, resistor R3 is shown as a fixed resistor in series with a potentiometer. For applications not requiring an exact scale factor (balanced modulator, frequency doubler, AGC amplifier, etc.) Pins 3 and 13 can be connected together and a single resistor from Pin 3 to ground can be used. In this case, the single resistor would have a value of 1/2 the above calculated value for R13. Step 2. The next step is to select RX and RY. To insure that the input transistors will always be active, the following conditions should be met: VX RX < I13, VY RY < I3 A good rule of thumb is to make I3RY ≥ 1.5 VY(max) and I13 RX ≥ 1.5 VX(max). The larger the I3RY and I13RX product in relation to VY and VX respectively, the more accurate the multiplier will be (see Figures 17 and 18). Let RX = RY = 10 k Ω, then I3RY = 10 V I13RX = 10 V since VX(max) = VY(max) = 5.0 V, the value of RX= RY = 10 kΩ is sufficient. Step 3. Now that RX, RY and I3 have been chosen, RL can be determined: K = 2RL RX RY I3 = 4 10 = 4 10 , or (10 k) (10 k) (1.0 mA) (2) (RL) Thus RL = 20 kΩ. Step 4. To determine what power supply voltage is necessary for this application, attention must be given to the circuit schematic shown in Figure 3. From the circuit schematic it can be seen that in order to maintain transistors Q1, Q2, Q3 and Q4 in an active region when the maximum input voltages are applied (VX′ = VY′ = 10 V or VX = 5.0 V, VY = 5.0 V), their respective collector voltage should be at least a few tenths of a volt higher than the maximum input voltage. It should also be noticed that the collector voltage of transistors Q3 and Q4 is at a potential which is two diode-drops below the voltage at Pin 1. Thus, the voltage at Pin 1 should be about 2.0 V higher than the maximum input voltage. Therefore, to handle +5.0 V at the inputs, the voltage at Pin 1 must be at least +7.0 V. Let V1 = 9.0 Vdc. Since the current flowing into Pin 1 is always equal to 2I3, the voltage at Pin 1 can be set by placing a resistor (R1) from Pin 1 to the positive supply: R1 = V+ –V1 2I3 15 V –9.0 V Let V+ = 15 V, then R1 = (2) (1.0 mA) R1 = 3.0 kΩ. Note that the voltage at the base of transistors Q5, Q6, Q7 and Q8 is one diode-drop below the voltage at Pin 1. Thus, in order that these transistors stay active, the voltage at Pins 2 and 14 should be approximately halfway between the voltage at Pin 1 and the positive supply voltage. For this example, the voltage at Pins 2 and 14 should be approximately 11 V. Step 5. For dc applications, such as the multiply, divide and square-root functions, it is usually desirable to convert the differential output to a single-ended output voltage referenced to ground. The circuit shown in Figure 22 performs this function. It can be shown that the output voltage of this circuit is given by: VO = (I2 –I14) RL And since IA –IB = I2 –I14 = 2IX IY I3 = 2VXVY I3RXRY then VO = 2RL VX′ VY′ 4RX RX I3 where, VX′ VY′ is the voltage at the input to the voltage dividers. Figure 22. Level Shift Circuit I2 I14 V2 V14 RO RO + - RL RL V+ VO The choice of an operational amplifier for this application should have low bias currents, low offset current, and a high common mode input voltage range as well as a high common mode rejection ratio. The MC1456, and MC1741C operational amplifiers meet these requirements. |
Ähnliche Teilenummer - MC1495 |
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Ähnliche Beschreibung - MC1495 |
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