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 ULTRA HIGH-SPEED ICs By Klaus Lehmann, Burr-Brown International GmbH 1 OPA660 Current Feedback Amplifier OPA622 Voltage Feedback Amplifier Out Out 2 OPA660 Straight Foward Amplifier +In Out 3 +In +In –In 4 5 QUASI-IDEAL CURRENT SOURCE In addition to their actual operation parameter transconductance, active electronic key components such as vacuum tubes, field effect transistors, and bipolar transistors demonstrate diverse negative parameters. In applying the socalled Diamond structure, the user can obtain an improved current source with reduced disturbance parameters, as well as a programmable transconductance independent of temperature. Standard applications for the Diamond current source (DCS) can be found in buffers, operational amplifiers with voltage or current feedback, and transconductance amplifiers. The DCS simplifies the design of electronic circuits with bandwidths of up to 400MHz and slew rates of 3000V/µs with a low supply current of several mA. 6 COMPONENT PARAMETER TYPICAL VALUE Triode Grid Bias Voltage Anode Bias Voltage Grid Current Anode Bias Current G/K Resistance A/K Resistance Trans Grid Action 0 to 10V 20 to 1kV nA to µA µA to A kΩ to MΩ kΩ to MΩ 1 to 20% N-J FET Gate Voltage D/S Voltage Gate Current D Bias Current G/S Resistance D/S Resistance Inverse Amplification 0 to –10V 0 to 100V fA to µA µA to A MΩ to GΩ kΩ to MΩ 1 to10% NPN Transistor Basis Voltage K/E Voltage Basis Current K Bias Current B/E Resistance K/E Resistance Inverse Amplification 0.5 to 0.8V 0.5 to 100V µA to mA µA to A kΩ kΩ 0.1 to 1% VOFF1 VOFF3 IBIAS1 IBIAS3 R12 R32 VR31 –2 to +2mV 0V nA to µA µA kΩ to MΩ kΩ <0.1% VOLTAGE-CONTROLLED CURRENT SOURCES For analog signal processing, especially current or voltage gain, previous electronic circuit techniques primarily used vacuum tubes, while today they use field effect or bipolar transistors. The triode illustrated in Figure 1 is representative of the various vacuum tubes, while the N-channel FET represents the FET variations (junctions, insulated gates, depletion, enhancements, P-channels, and N-channels), and a NPN transistor represents the range of bipolar transistors. Triodes, N-J FETs, and NPN transistors are compared with the Diamond current source (DCS). The common elements of all of these active elements are a relatively high-impedance input electrode 1 (grid, gate, basis), a low-impedance © 1993 Burr-Brown Corporation AB-180 DCS TABLE I. Typical Disturbance Parameters of the VoltageControlled Current Sources. Printed in U.S.A. May, 1993 describes the change of the output signal (VOUT) dependent upon the input signal (VIN). input and output electrode 2 (cathode, source, emitter), and a high-impedance output electrode 3 (anode, drain collector). Thus all of these elements can be treated as special voltage-controlled current sources (VCCS = Voltage-Controlled Current Source). The limitation “special” refers to the low-impedance input and output electrode 2. The most important relation between the electrodes 1, 2, and 3 is the transconductance gm. For instance, the transconductance Triade 3 1 G –gm K Ia = K (Vgk + gm = dIa dVgk + G 2 Id = β (1 + λVds) (Vgs – Vp )2 gm = dId dVgs gm = = 2β (1 + λVgs) (Vgs – Vp ) dTc dVbe Vbc VAF = – Is VT –gm 2 2 Vbe VAR ) (eVbe /VT – eVbe /VT) I3 = V12 x gm gm = 2K eVbe /VT Ig VT K = 0.81; VT = 25.9mV Is = 1.58E –16; VAF = 66; VAR = 3 β = 2.258E –3; λ = 21.31E–3; Vp = –2 K = 0.001; D = 0.05 Ia (mA) Vgk 30 25 20 15 10 5 –1V –2V –3V –4V –5V Vak Ia Vak 40V 60V 80V 20V 100V –Vgk gm 6 4 60V 80V 1 2 3 (V) –5 –4 –3 –2 –1 5 Vds +5V +2.5V –Vgs –1.5 –1 gm Vds +5V +2.5V 3 1 (V) (mA) (mA) 10 10 8 8 6 6 4 4 2 2 (V) –2 (mA/V) (mA/V) –1 3 10 8 4 –I3 1V...5V –V12 (mV) Vbe 0.72 (mA) Ig 2.4mA 1.2mA 0.6mA 0.3mA –5 +V12 +20 +40 +60 (mV) (mA) (mA/V) 160 5V 3V 1V 120 40 Vbe 0.80 –60mV (mA) 80 0.76 (V) –20mV –60 –40 –20 (gm) 100 +V32 +3 +5 –10 –I3 1 (V) gm Vce +2 +10 Vce 200 –0.5 +1 –10 (V) 5 +I3 300 6 –1 –40mV Ic 400 2 –1.5 2 –2 +20mV –5 Vce 0.2 0.4 0.6 0.8 4 –Vgs (V) –3 0.786V V12 +40mV +5 –V32 0.804V 2 –0.5 (mA/V) 1 20V 4 Id 2 100V 4 Vds +60mV Vbe 0.828V 0.822V 0.815V 6 –1.1V (mA) +10 8 –Ø.73V 2 4 40V Vgs ±ØV –Ø.21V –Ø.45V 8 (V) –2 +I3 Ic 10 (mA) 12 10 8 6 4 2 (V) –5 –4 –3 –2 –1 Vak (mA) Id 10 100 150 200 (V) 50 Ic = Is (1 – 1 –gm E 2 3 3 1 B Id = βVds (1 + λVds) [2 (Vgs – Vβ ) – Vgs] K Vgk + DVak DCS C –gm gm 3 DVak) /2 3 3 1 2 Vgk < Ø (Vgk + DVak) > Ø –Vgk NPN - Transistor D (1) To operate each VCCS, it is necessary to adjust the DC quiescent current or voltage individually (see Figure 1). N - J - FET A (mA) VOUT = VIN x gm x ROUT 0.84 (V) –V12 Ig 2.4mA 1.2mA 0.6mA 0.3mA +V12 (mV) 60 40 20 – + 20 40 60 (mV) FIGURE 1. Comparison Between Voltage-Controlled Current Source (VCCS) and Diamond Current Source (DCS). 2 Input Signal Real VCCS Ideal VCCS Input Compensation Output Compensation Output Signal VOUT 3 VOFF1 – + 1 V'OFF1 V'r31 + – + – 1' R'12 IBIAS1 + – V'OFF3 VOFF3 gm 2' VIN – + 3' I'BIAS1 R'32 RL I'BIAS3 R'S 1/ gm IBIAS3 2 FIGURE 2. Internal and External Substitute Circuitry of a Voltage-Controlled Current Source. Figure 2 illustrates the inner and outer substitute circuitry of a voltage-controlled current source VCCS. According to the circuitry, the VCCS (1, 2, 3) consists of an inner ideal VCCS (1', 2', 3') with transconductance gm and a row of inner disturbance parameters (V', I' , R'), which determine, among other things, the adjustment of the DC point. Table I shows a rough overview of the disturbance parameters. Almost all disturbance parameters are subject to tolerances between units and show dependent temperature behavior. mond circuit, illustrated in Figure 4, opens up the possibility of implementing the quasi-ideal VCCS [2]. In the ideal case, in which NPN and PNP transistors are identical, the disturbance parameters V0FF1', VOFF3', IBIAS1', and IBIAS3' disappear. But in real circuits, of course, this is not the case. The remaining parameter values are, however, much smaller in comparison with a conventional VCCS (compare VCCS with DCS in Figure 1 and Table I). In the modulation range being examined, from I3 = ±10mA, the transconductance varies from 120 to 160mA/V as opposed to 0 to 350mA/V. This means that the improved VCCS (designated DCS from now on) causes a reduction in signal distortion. Figure 2 also illustrates the correction parameters (VOFF1, VOFF3, IBIAS1, and IBIAS3), which are required primarily to compensate the internal disturbance parameters. The correction parameters, however, do not correct the effects of the internal disturbance parameters (R'12, R'32) and the output voltage feed-through V'r31. Roughly stated, at least 50% of the design time for electronic circuit techniques goes toward dealing with the problem of compensation. Thus in complex circuits, the connection between the function parameter gm and the various disturbance parameters requires more and more modifications in circuit variations. If a VCCS without disturbance parameters was available for users, the huge variety of electronic circuit techniques could be reduced. 1 VIN 2 OTA 3 B VOUT R2 Rf FIGURE 3. Operational Amplifiers as Series Connection Between OTA and Buffer. THE “IDEAL” CURRENT SOURCE The macro element operational transconductance amplifier (OTA) and operational amplifier (OA) contain circuit parts for reducing the previously mentioned disturbance parameters. The feedback operation necessary with these amplifiers— i.e. the application of a control loop with its unavoidable delay time (phase delays)— causes significantly reduced time and frequency domain performance compared to the VCCS. Straight-forward amplifiers are thus more widebanded than feedback amplifiers. An operational amplifier OA, as shown in Figure 3, consists of the series connection of an OTA with a buffer B. The OTA is a voltage-controlled current source VCCS, in which the electrode 2 can be used “only” as a high-impedance input. Because of this distinction, the OTA can only be used with an external feedback loop. In contrast to conventional operational amplifiers with voltage feedback as shown in Figure 3, the current-feedback OA contains an OTA with low-impedance input and output 2—i.e. the previously represented “ideal” VCCS. The Dia- VCC I'Q 3 3 IO 1 1/ 6 1/ 2 3 6 IO 3 3 I'Q VEE FIGURE 4. VCCS with Diamond Structure. 3 PROGRAMMABLE TRANSCONDUCTANCE Conventional VCCSs allow the transconductance to be adjusted depending upon the quiescent current. In the DCS, the transconductance is adjusted primarily with the current sources I'Q (see Figure 4). For this adjustment, one effective method is to create a current source control (Figure 5). are available for the DCS (Figure 7): Buffer (B), CurrentFeedback Transconductance Buffer (TB), Transconductance Amplifier (TA), Direct-Feedback Transconductance Amplifier (TD), Current-Feedback OA (TCC), and VoltageFeedback OA (TCV). Using the resistor RQ, the quiescent current I'Q or IQ and thus the transconductance gm can be fixed. The temperature function of gm (due to VT = f(T)) is compensated for by corresponding variations of IQ. For RQ → ∞, IQ → 0 and gm → 0, and VCCS is switched off. In contrast to the conventional VCCS, the DCS functions in two quadrants at the input and in four at the output. In the VCCS, the transconductance is fixed by the choice of DC points within the usable modulation range, while the transconductance in the DCS is largely independent of the modulation and can be adjusted with the external resistor RQ. OUTLOOK To characterize typical dynamic coefficient values (Table II), a developed DCS including a SOI package was simulated in the circuit shown in Figure 8. Burr-Brown brought this DCS onto the market as OPA660. LITERATURE [1] Ross, D.G. et al; IEEE Journal of solid-state circuits 86, vol. 2, p. 331. [2] Lehmann, K; Elektronik Industrie 89, vol. 5, p. 99. Strom-oder Spannungs-Gegenkopplung? (Current or Voltage Feedback? That’s the question here.) gm = dI3/dV12 is negative for all VCCSs. In contrast, the transconductance gm = dI3/dV12 of the DCS is positive. As previously mentioned, the following standard applications VCC 3 1 1 1 VT I'Ø = RØ I'O I'O gm = I'O I'O 2KIØ VT = 1 2K X 18I'Ø RIN 1 +gm 2 ROUT2 +gm 2 VOUT VT 2K X 18 X In (10) gm = 10 VIN In (1Ø) 1 RØ FIGURE 8. Circuit for Recording the Dynamic Characteristic of a TCC with DCS. 1 RQ VEE FIGURE 5. Current Source Control with Adjustable Bias Current. +V 4 3 1 (mA/V) 2 6 RQ 100 –V (gm) 10 RQ 1 0.1 1 0.1Vp-p f–3dB (MHz) 6Vp-p f–3dB (MHz) 4Vp-p SR (V/µs) 1.4Vp-o DG (%) 5MHz DP (Degrees) 2.4 1.2 0.6 0.3 400 240 140 80 330 200 100 55 2850 1750 800 420 –0.07 –0.06 –0.05 –0.03 –0.05 –0.06 –0.10 –0.19 TABLE II. Typical Dynamic Values of a TCC with DCS Corresponding to the Circuit in Figure 8. +gm 1000 5 IQ (mA) 10 (Ω) FIGURE 6. The Relations gm = f(RQ) and Block Diagram of the DCS. 4 B TB 3 VIN 1 1 VIN +gm 2 RIN VB = 1/[1 + 1/(gm x RIN)] = 1 3 Ri2 = 1/gm +gm VTB = 1/[1 + 1/(2gm x RIN)] = 1 2 RIN V'OUT TA TA V'OUT VOUT 3 VIN 1 +VTA = ROUT/(RIN + 1/gm) = ROUT/RIN 3 1 ROUT +gm 2 RIN Ri2 = 1/2gm VOUT +Ri3 = ROUT ROUT +gm –VTA = +VTA 2 RIN –Ri3 = ROUT VIN TC TC VOUT 3 VIN 1 1 +gm +gm ROUT RIN ROUT 3 3 1 +gm 1 +gm 2 ROUT ROUT 1 2 ROUT 2RIN RIN (ROUT/2 – 1/(2gm) RIN + 1/(2gm) 2 –VTCC = – = ROUT 2 ROUT RIN ROUT RIN VOUT TCV 1 +gm +gm 3 +gm 1 =– +gm 3 VIN 2RIN +VTCC = 1 + VIN VOUT 1 +gm 2 RIN TCV RIN + 1/(2gm) Ri3 = ROUT – TCC 2 RIN ROUT 2 TCC 1 =1+ ROUT/2 – 1/(2gm) –VTC = VIN VIN ROUT/2 + RIN RIN + 1/(2gm) 3 2 RIN +VTC = VOUT ROG 1 2 ROUT 2 +gm 1 VOUT 2 RIN ROG 1 3 +VTCV = 1 + 2 –VTCV = – RIN VIN FIGURE 7. Standard Applications with the DCS. 5 ROUT VOUT ROUT RIN ROUT RIN