APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 BUILDING A 400MHz WIDE-BAND DIFFERENTIAL AMPLIFIER: IT'S A BREEZE WITH THE DIAMOND TRANSISTOR OPA660. By Christian Henn and Ernst Rau, Burr-Brown International GmbH In radio frequency (RF) technology, signals from oscilloscopes, monitors, transient recorders, and many other devices are usually connected to sensors and generators via coaxial lines. In any transmission, however, interference voltages caused by differences in potential between the sender and receiver and by electromagnetic interference distort the results, particularly when the signals being transmitted are sensitive and wide-band. Designers of this type of transmission system need shielded, symmetrical transmission lines and input differential amplifiers with high common-mode rejection, which bring home the signals without humming or radio interference. 1 current source converts the symmetrical input voltage VIN either into an output current or into the asymmetrical output voltage VOUT when a voltage drop is present at the external resistor ROUT. VIN and VOUT are related as follows: VOUT = VIN • gm • ROUT, where gm is the transconductance of the operational transconductance amplifier (OTA). The buffer following the input amplifier decouples the low-impedance load resistor from the high-impedance OTA output. Instead of symmetrical signal excitation, many applications use the type of transmission path shown in Figure 2. A single-ended signal voltage VIN drives an asymmetrical coaxial cable terminated on both sides. In this structure as well, the symmetrical differential amplifier input rejects interference voltages superimposed on the signal. Designing this type of differential amplifier used to be quite a chore, involving extensive and complicated hardware. But the development of new, monolithic ICs such as the Diamond Transistor OPA660 has changed all that. The OPA660 makes it easy to design a 400MHz differential amplifier offering –60dB common-mode rejection at a 1MHz frequency. This amplifier uses an open-loop amplifier structure with two identical high-impedance inputs and no feedback. The parameters such as wide bandwidth, stable operation, and excellent pulse processing, common-mode rejection, and harmonic distortion let the performance speak for itself. INSTRUMENTATION AMPLIFIER WITH FEEDBACK OTAs and buffers have conventionally been designed using differential amplifiers as shown in Figure 3. The feedback path from the op amp output over R4 generates a relatively low-impedance inverting input, which is equal to the R3 resistor value. Inserting the buffer amplifier, BUF2, converts the low-impedance input to high impedance, while inserting the buffer amplifier, BUF1, optimizes the input symmetry and thus the common-mode rejection at DC and vs frequency. BASIC TRANSMISSION STRUCTURES Figure 1 shows a symmetrical transmission path with signal voltage VS and cable termination resistors RIN and Rt. A symmetrical voltage source normally uses amplifiers with complementary outputs or transformers to balance or adapt the circuits. The relatively high-impedance input resistor Rb limits the input potential drift through the input bias currents (IBIAS), and the symmetrical differential amplifier input rejects interference voltages superimposed upon the input signal and its reference potential. The voltage-controlled The gain is R4/R3 during signal excitation at the inverting input and 1 + R4/R3 during signal excitation at the noninverting input. A divider is inserted between R1 and R2 to compensate for these differing gains. Buffer 1 also synchronizes the signal delay times of the two inputs, which is important for good common-mode rejection at high frequencies. To achieve high common-mode rejection over frequency, it is important that the gain curve of the two input buffers be as identical as possible. +VCC RIN IBIAS gm VIN Rt VIN VOUT VOUT Buffer OTA IOUT IBIAS ROUT Rb RLOAD –VCC FIGURE 1. Basic Structure of a Symmetrical Transmission Path. © 1993 Burr-Brown Corporation AN-188 Printed in U.S.A. November, 1993 2 3 4 5 6 In addition to requiring more hardware, this type of system also has smaller bandwidth than the open-loop amplifier shown in Figure 2 due to the delay time in its amplifier feedback loop (phase shift). both worlds, offering better bandwidth than a normal openloop amplifier, excellent pulse responses down to rise/fall times of 1ns, and reduced hardware. The basic concept is shown in Figure 4. A SYNTHESIS: OPEN-LOOP AMPLIFIER USING THE DIAMOND TRANSISTOR The gain can be determined according to the following equation: R OUT V OUT = V IN R + 2 /gm The open-loop amplifier using the Diamond Transistor OPA660 and buffer amplifier BUF601 combines the best of E +VCC RIN IBIAS gm Rt VIN VIN VOUT VOUT Buffer OTA IOUT IBIAS ROUT RLOAD –VCC FIGURE 2. Signal Transmission Using an Asymmetrical Coaxial Cable and a Signal Voltage Referred to Ground. BUF600 R1 R2 BUF 1 OPA622 RIN VOUT Rt VIN VIN OPA R3 R4 RLOAD BUF 2 BUF600 FIGURE 3. Instrumentation Amplifier with Feedback. ROUT BUF601 VOUT VOUT BUF 2 IOUT DT gm RLOAD OPA660 RIN VIN Rt VIN RE BUF 1 FIGURE 4. Wide-Band Open-Loop Amplifier. 2 Since the actual symmetrical structure of the circuit layout greatly effects the bandwidth and common-mode rejection, a demo board was used to determine the characteristic transmission parameters that this configuration shows in practice. Figure 5 illustrates the demo board in detail. The silkscreen and layout tips can be extremely useful in designing your own layouts. TEST RESULTS The amplifier stage is set to a gain of +4 at an R9 of 240Ω and R8 of 43Ω. The total gain from input to output, including the output divider R11/RL, is +2. Figure 6 illustrates the frequency response of the two inputs In+ and In–. The –3dB frequency (fg) is 400MHz. Figure 7 shows the impact of the capacitor C5 on the bandwidth. The OPA660 contains a transconductance amplifier nicknamed the Diamond Transistor and a buffer called the Diamond Buffer in an 8-pin package. The Diamond Transistor itself consists of a buffer identical to the Diamond Buffer, followed by a current mirror. On the output side, the buffers are connected to each other via the resistor R8, forming the differential input stage. When the input voltage is differential, a current flows through R8, is reflected in high-impedance form to Pin 8, and produces the output voltage at R9. To drive low-impedance transmission lines or input resistors, the buffer amplifier BUF601 decouples the relatively high-impedance output of the differential amplifier. Both inputs and the output are laid out for 50Ω systems, but they can also be adapted to other characteristic impedances by replacing the resistors R3, R7, and R11. Capacitor C5 parallel to R8 compensates the parasitic capacitor at Pin 8 of the OPA660, thus expanding the achievable bandwidth. The common-mode gain over frequency curve shown in Figure 8 demonstrates the rejection of interference voltages on both input voltages. The interference remains less than –18dB over the entire bandwidth, starting at a commonmode gain of –68dB. While the 400MHz differential amplifier amplifies differential signals by 4, the common-mode noise of the same frequency that appears at the output is multiplied only by 0.125. Table I lists the common-mode gain for several frequency levels. fIN CG 1MHz 10MHz 100MHz –60dB –45dB –23dB TABLE I. Several Common-Mode Gains. The harmonic distortions shown in Figure 9 and Table II for two different output voltages over frequency are outstanding parameters for a 400MHz differential amplifier and prove that the OPA660 and BUF601 provide excellent reproduction of wide-band input signals even without feedback. Furthermore, the low noise voltage density of 7.7nV/√Hz makes it possible to process even very small signals. The resistors, R4, R6, and R10, located at the front of the circuit in series to the high-impedance inputs, make it possible to set the frequency response at the end of the bandwidth for a flat response. The quiescent current of the OPA660 is ±20mA at an R16 of 560Ω. +5V CB = 2.2µF || 10nF R9 240Ω CB +5V CB 8 7 R3 51Ω OPA660 R4 150Ω In+ R6 150Ω In– R10 150Ω 3 DT 1 4 8 R11 51Ω BUF601 5 5 CB DB R7 51Ω 4 1 6 R16 560Ω R8 43Ω C5 18pF CB –5V FIGURE 5. Circuit Diagram of the Demo Board. 3 2 –5V IQ = ±20mA (OPA660) IQ = ±6mA (BUF601) OUT HIGH PROCESSING POWER, LOW POWER REQUIREMENTS f The most important job of a differential amplifier is to reject common-mode interference arising during the transmission of analog signals. The 400MHz differential amplifier using the OPA660 impressively demonstrates how easy it now is to design wide-band input amplifiers for test devices, monitors, transient recorders, and other RF devices. While achieving excellent parameters for bandwidth, common-mode rejection, and frequency response, the OPA660 and BUF601 also offer such low power consumption that the entire differential amplifier can be powered from a separate battery supply—a truly compact, high-performance alternative. 10MHz 10MHz 10MHz 10MHz Gain (dB) 1Vp-p 1st Harmonic 1Vp-p 2nd Harmonic 2Vp-p 1st Harmonic 2Vp-p 2nd Harmonic –61dB –64dB –57dB –55dB Common Mode Gain (dB) –10 –20 –30 –40 –50 –60 0 +VIN (Pin 5) –70 300k –10 –VIN (Pin 3) 1M 10M Frequency (Hz) 100M 1G FIGURE 8. Common-Mode Gain. –20 –85 10M Frequency (Hz) 100M 1G Harmonic Distortion (dBc) 1M FIGURE 6. Frequency Responses of the Inputs In+ and In–. 10 C5 = 18pF 0 Gain (dB) HARMONIC DISTORTION TABLE II. Harmonic Distortion. 10 –30 300k VOUT –75 2f 1Vp-p –65 1f 1Vp-p –55 2f 2Vp-p –45 1f 2Vp-p C5 –35 1M –10 FIGURE 9. Harmonic Distortion. –20 –30 300k 10M Frequency (Hz) 1M 10M Frequency (Hz) 100M 1G FIGURE 7. Impact of Capacitor CS on the Bandwidth. 4 100M