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NE5209D Datasheet(PDF) 5 Page - NXP Semiconductors |
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NE5209D Datasheet(HTML) 5 Page - NXP Semiconductors |
5 / 15 page Philips Semiconductors Product specification NE/SA5209 Wideband variable gain amplifier 1990 Aug 20 5 NE5209 APPLICATIONS The NE5209 is a wideband variable gain amplifier (VGA) circuit which finds many applications in the RF, IF and video signal processing areas. This application note describes the operation of the circuit and several applications of the VGA. The simplified equivalent schematic of the VGA is shown in Figure 2. Transistors Q1-Q6 form the wideband Gilbert multiplier input stage which is biased by current source I1. The top differential pairs are biased from a buffered and level-shifted signal derived from the VAGC input and the RF input appears at the lower differential pair. The circuit topology and layout offer low input noise and wide bandwidth. The second stage is a differential transimpedance stage with current feedback which maintains the wide bandwidth of the input stage. The output stage is a pair of emitter followers with 50 Ω output impedance. There is also an on-chip bandgap reference with buffered output at 1.3V, which can be used to derive the gain control voltage. Both the inputs and outputs should be capacitor coupled or DC isolated from the signal sources and loads. Furthermore, the two inputs should be DC isolated from each other and the two outputs should likewise be DC isolated from each other. The NE5209 was designed to provide optimum performance from a 5V power source. However, there is some range around this value (4.5 - 7V) that can be used. The input impedance is about 1k Ω. The main advantage to a differential input configuration is to provide the balun function. Otherwise, there is an advantage to common mode rejection, a specification that is not normally important to RF designs. The source impedance can be chosen for two different performance characteristics: Gain, or noise performance. Gain optimization will be realized if the input impedance is matched to about 1k Ω. A 4:1 balun will provide such a broadband match from a 50 Ω source. Noise performance will be optimized if the input impedance is matched to about 200 Ω. A 2:1 balun will provide such a broadband match from a 50 Ω source. The minimum noise figure can then be expected to be about 7dB. Maximum gain will be about 23dB for a single-ended output. If the differential output is used and properly matched, nearly 30dB can be realized. With gain optimization, the noise figure will degrade to about 8dB. With no matching unit at the input, a 9dB noise figure can be expected from a 50 Ω source. If the source is terminated, the noise figure will increase to about 15dB. All these noise figures will occur at maximum gain. The NE5209 has an excellent noise figure vs gain relationship. With any VGA circuit, the noise performance will degrade with decreasing gain. The 5209 has about a 1.2dB noise figure degradation for each 2dB gain reduction. With the input matched for optimum gain, the 8dB noise figure at 23dB gain will degrade to about a 20dB noise figure at 0dB gain. The NE5209 also displays excellent linearity between voltage gain and control voltage. Indeed, the relationship is of sufficient linearity that high fidelity AM modulation is possible using the NE5209. A maximum control voltage frequency of about 20MHz permits video baseband sources for AM. A stabilized bandgap reference voltage is made available on the NE5209 (Pin 7). For fixed gain applications this voltage can be resistor divided, and then fed to the gain control terminal (Pin 8). Using the bandgap voltage reference for gain control produces very stable gain characteristics over wide temperature ranges. The gain setting resistors are not part of the RF signal path, and thus stray capacitance here is not important. The wide bandwidth and excellent gain control linearity make the NE5209 VGA ideally suited for the automatic gain control (AGC) function in RF and IF processing in cellular radio base stations, Direct Broadcast Satellite (DBS) decoders, cable TV systems, fiber optic receivers for wideband data and video, and other radio communication applications. A typical AGC configuration using the NE5209 is shown in Figure 3. Three NE5209s are cascaded with appropriate AC coupling capacitors. The output of the final stage drives the full-wave rectifier composed of two UHF Schottky diodes BAT17 as shown. The diodes are biased by R1 and R2 to VCC such that a quiescent current of about 2mA in each leg is achieved. An NE5230 low voltage op amp is used as an integrator which drives the VAGC pin on all three NE5209s. R3 and C3 filter the high frequency ripple from the full-wave rectified signal. A voltage divider is used to generate the reference for the non-inverting input of the op amp at about 1.7V. Keeping D3 the same type as D1 and D2 will provide a first order compensation for the change in Schottky voltage over the operating temperature range and improve the AGC performance. R6 is a variable resistor for adjustments to the op amp reference voltage. In low cost and large volume applications this could be replaced with a fixed resistor, which would result in a slight loss of the AGC dynamic range. Cascading three NE5209s will give a dynamic range in excess of 60dB. The NE5209 is a very user-friendly part and will not oscillate in most applications. However, in an application such as with gains in excess of 60dB and bandwidth beyond 100MHz, good PC board layout with proper supply decoupling is strongly recommended. Q1 Q2 VCC Q3 Q4 Q5 Q6 VBG BANDGAP REFERENCE OUTA OUTB Q8 Q7 A1 VAGC 0–1V I1 I2 I3 50 Ω 50 Ω R1 R2 R3 R4 INA INB + – SR00238 Figure 2. Equivalent Schematic of the VGA |
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